Aerodynamics The Wing Is the Thing

Transcription

1 Page 1 Chapter Two erodynamics The Wing Is the Thing Courtesy Cessna ircraft Co. We often use mechanical equipment without completely understanding how it works. Don t take my word for that. Go stand at a hardware store some Saturday morning, and listen to people talk about thingamajigs, doohickies, and gizmos while waving their hands, hyperventilating, and attempting to describe for the clerk what isn t working. Do you think unt Maude understands why her toaster toasts? Do you think your boss understands why his or her personal computer can do what it does? s a young bachelor my parents gave me a vacuum cleaner for my birthday. Several months later mom called and asked, re you having trouble finding bags for your vacuum cleaner? I said, ags? What bags? How was I to know the thing needed bags? Technological ignorance has its advantages, but not when you re up in the air. You don t need a Ph.D. in aerodynamics to be a pilot, but moderate-to-decent understanding of why an airplane stays airborne will prove very helpful and life sustaining. s one aviator elegantly put it, The wing is the thing. It s the thing that performs the magic; the pilot simply manages the show. May the Four Forces e With You Yoda, the transcendental handheld philosopher from the Star Wars movie trilogy, frequently dispatched Luke Skywalker with the benediction, May The Force be with you. In aviation, there are Four Forces and they are always with us, whether Yoda or your flight instructor intervenes or not. How am I going to land without grinding off the weight arrow? THRUST Fig. 1 LIFT WEIGHT Under Yoda s tutelage, Luke extracted from Mother Nature the mightiest of her secret powers. eronautical engineers taller, without green complexions, blanketed with pocket protectors do the same, designing airplanes to create a compromise among The Four Forces. This results in the ability of your trusty airplane to fly. Compared to crafting this compromise, settling the Middle East conflict is novice-level work. The Four Forces lift, weight, thrust and drag are present any and every time a plane is airborne. Look at Figure 1, which shows the action of The Four Forces. Of course, enormous arrows don t really protrude from the airplane. I know this will disappoint those of you who still FOUR FORCES CTING ON N IRPLNE IN FLIGHT DRG When the airplane is in flight, there are always four forces acting on it: lift, thrust, weight and drag.

2 2 Rod Machado s Private Pilot Handbook Professor ob: While hiking on Mount Pinitubo during the last eruption, Professor ob had an aerodynamic brainstorm when he fell into a lava pit. He reasoned that if your turned the airplane upside down and backwards, the drag vector would pull it forward as you reduced thrust. Reducing lift would expect the states to be colored blue and red and have lines drawn around their borders as you fly over them, but you ll get used to it. The arrows do serve to show that what we ve got here is a highly competitive new game four-way tug-of-war. Your job, as pilot, is to manage the resources available in order to balance The Four Forces. Learning to fly is really learning to manage The Four Forces. lmost everything you do in the cockpit will result in making a change to one or more of these forces. Let s see what they re all about. Lift is the upward-acting force created when an airplane s wings move through the air. Forward movement produces a slight difference in pressure between the wings upper and lower surfaces. This difference becomes lift. It s lift that keeps an airplane airborne. I discovered how lift works at four years of age, during my very first visit to church. The collection plate passed in front of me and I picked out a few shiny items. My grandfather chased me around the pew and I thought, Wow, church is fun! Picking me up by my sweater, grandpa held me suspended four feet off the ground and toted me outside. It was the lift from grandpa s arm, precisely equaling my weight, which kept me airborne. Wings do for the airplane what grandpa s arm did for me provide the lift to remain aloft. DRG allow the weight vector to pull the airplane skyward. Once again, we see the perils of skipping one s medication, even for a day. Professor ob Chief Paleoaerodynamicist WEIGHT LIFT Weight is the downward-acting force. It s the one force pilots control to some extent by choosing how they load the airplane. With the exception of fuel burn, the airplane s actual weight is difficult to change in flight. Once airborne, you should not be burning cargo or acquiring extra passengers (or losing them). Unexpected discharge of passengers while in flight is a violation of some F rule, so please don t do it. In unaccelerated flight (when the airplane s speed and direction are constant), the opposing forces of lift and weight are in balance. Thrust is a forward acting force produced by an engine-spun propeller. For the most part, the bigger the engine (meaning more horsepower) the greater the thrust produced THRUST Produced by engine power THRUST Fig. 2 THRUST and the faster the airplane can fly up to a point. Forward movement always generates an aerodynamic penalty, called drag. Drag pulls rearward on the airplane and is simply the atmosphere s molecular resistance to motion through it. In plain English (which pilots and engineers rarely use), it s wind resistance. Few things are free with Mother Nature. s Confucius might say, Man who get something for nothing not using his own credit card. Thrust causes the airplane to accelerate, but drag determines its final speed. s the airplane s velocity increases, its drag also increases. Due to the perversity of nature, doubling the airplane s speed actually quadruples the drag. Eventually, the rearward pull of drag equals the engine s thrust, and a constant speed is attained. My high school VW ug knew these limits well (it s called a ug because that s the largest thing you can hit without totaling the car). The ug s forward speed is limited by its engine size. With four little cylinders (three of which worked at any one time), this VW simply wouldn t go faster than 65 mph. Figure 2 shows the results of maximum thrust meeting the equal and rearward pull of drag at this speed. Maintaining a slower speed requires less power, since less drag exists. t any speed less than the maximum forward speed of the car, excess thrust (horse- N UTOMOILE'S THRUST ND DRG I'm Rod's car. My horn doesn't blow but the tires do! The hood doesn't collapse but the steering wheel does. DRG Caused by resistance of air molecules DRG constant velocity is attained when the automobile's engine thrust equals the drag produced by wind resistance. t full throttle, the car's top speed is limited by the maximum thrust the engine can produce (same for airplanes too).

3 LIFT Chapter 2 - erodynamics: The Wing is the Thing 3 I'm Rod's car. My lue ook value varies with the amount of gas in my tank. Thrust provided by engine THRUST The road supplies an upward force N UTOMOILE'S FOUR FORCES WEIGHT Lift provided, by the road, o acts 90 to car's motion Drag due to wind resistance DRG Weight of car pushing down on the road on the car at a 90 degree angle to the direction of travel. Lift (developed by the wings) supplies a similar force on the airplane at a 90 degree angle to the direction of travel. Fig. 3 power) is available for other uses, such as accelerating around other cars or perhaps powering a portable calliope if you are so inclined. The same is true of airplanes. t less than maximum speed in level flight, there s power (thrust) to spare. Excess thrust can be applied to perform one of aviation s most important maneuvers the climb. Climbs One of aviation s biggest misconceptions is that airplanes climb because of excess lift. This is similar to believing that putting hand lotion in your airplane s fuel tank will make your landings smoother, softer and younger looking. irplanes climb because of excess thrust, not excess lift. Let s use my car as an example to see how thrust, not lift, is responsible for the climbing action of an airplane. Figure 3 shows my car sprouting more arrows than the hat Custer wore at the Little ig Horn. Notice that the car s weight is offset by the upward push of the road on the tires. In effect, the road simulates the lift of an airplane s wings. In other words, if the road were quicksand, there would be little upward push (lift) and the car would slowly be absorbed by this evil liquid. Figure 3 also shows the lifting force acting at a 90 degree angle to the car s motion (it s the only way this lift can possibly act since the road pushes directly against the car s tires). Weight, on the other hand, always acts downward toward the center of the earth (pointing toward a Kung-Fu school in China somewhere). Now let s allow the car to move up a hill (climb). Figure 4 shows the forces on the climbing car. Notice that the lifting force is still perpendicular to the road, thus 90 degrees to the car s motion (arrow ). Lift appears tilted rearward slightly, because the road is inclined upward. The total weight of the car, however, still acts directly downward, toward that Kung Fu school (arrow ). No matter how steeply the road tilts, weight will always point toward the center of the N UTOMOILE'S FOUR FORCES IN CLIM It is engine power (F) that pulls the car uphill, not the lift from the road (). The steeper the hill the more the force of weight () acts aft (D), pulling in the direction of drag (E). Lift () still acts opposite weight (C), which is acting 90 degrees to the motion (the path) along the road. F THRUST DRG Steep Hill C Weight opposite lift Weight D 90 o TOTL WEIGHT Road lift 90 to motion Scotty, I need more power! earth (and the author s decision is final on that!). We can think of weight as two separate forces. Part of the car s weight pushes on the road in a perpendicular direction (arrow C). smaller part acts rearward (arrow D). Do you see what s happening here? In a climb, some of the car s weight starts acting rearward, in the direction of drag. nything that pulls aft and impedes acceleration even if it s the car s weight acts like drag. The steeper the hill, the larger the rear- Fig. 4 E SINGLE FORCE CN E COMINTION OF SMLLER FORCES We will think of arrows as showing the direction a force (a push) is applied. When ud and Ed simultaneously push on the block in different directions, the block moves in a new direction that results from the combined forces. This new direction (Resultant Force #3) is simply a combination of the smaller forces (Force #1 and Force #2). The weight that pulls a car downward can be thought of as two individual forces when the car's on an incline. I'm ud I'm Ed FORCE #2 lock moves in this direction as a result of Ed's push. The block now moves in this direction as a result of the combined motions of both ud and Ed. lock moves in this direction as a result of ud's push. FORCE #1 RESULTNT FORCE #3 Steep Hill 90 o On an incline the car's total weight can be thought of as being a combination of two individual weights: one that pushes directly on the road (Force #1) and one that pulls the car backward along the road (Force #2). FORCE #1 FORCE #2 TOTL WEIGHT

4 4 Rod Machado s Private Pilot Handbook ward acting forces. (If you re having trouble with vectors, see the accompanying sidebar at the bottom of page 3.) Here s what you ve been waiting for: The upward push of the road on the car (arrow ) is equal to the car s weight on the road (arrow C). In other words, lift still equals weight, even in a climb. Part of the weight, however, now acts like drag (arrow D), which really is a drag, because it gets added to the wind resistance. s we ve already learned, thrust is the force that overcomes drag. The forces acting on an airplane during a climb are similar to those of the car (Figure 5), the only major difference being that you (the pilot) choose the slope of the hill you climb. This is done using the elevator control in the cockpit (more on the elevator control later). s you can see, it s excess thrust, not lift, that allows an airplane to climb. Given this very important bit of knowledge, you ll now understand why smaller airplanes with limited power can t climb at steep angles like the lue ngels do at airshows. Let s go back to the automobile and climb a steep hill (Figure 6). The maximum forward speed of our car POWER ND CLIM NGLE Full Throttle 50 MPH 40 MPH 65 MPH Full Throttle Full Throttle ones! Can you give me more power? Even with full power, the car starts to slow down as the hill steepens. Eventually the car will come to a stop if the hill becomes too steep. In an airplane, you don't try to climb too steep a hill or you might slow down to the point where your wings can't develop the necessary lift to remain in flight. C Fig. 6 N IRPLNE'S FOUR FORCES IN CLIM Similar to the automobile, it is engine thrust (F), not extra lift () from the wings, that pulls an airplane up its "pilot-made" hill. The steeper the angle of climb (the hill) the more the force of weight () acts aft (D). That portion of the weight (D), acting in the direction of drag (E), pulls the airplane aft and acts just like drag. Lift () still acts opposite the portion of weight (C) that acts 90 degrees to the flight path (which is also the relative wind). F C D on a level road with full power is 65 mph (Car ). s we move up a hill (Car ) our speed drops to 50 mph. n even steeper hill slows the car to 40 mph (Car C). The limited horsepower of the car s engine simply can t match the drag caused by wind resistance plus rearward-acting weight as the hill steepens, so the car slows. bigger engine or redesign of the car to produce less wind resistance are the only options that will help. The same analysis works, up to a point, for an airplane attempting to climb a hill in the air (Figure 7). Let s say your airplane has a maximum speed of 120 knots in straight and level flight with full throttle (irplane ). (irplane throttles are similar to automobile throttles except that they re hand operated. You push in for more power and pull out for less.) pplying slight back pressure on the elevator control points the airplane s nose upward (irplane ). This causes the airplane to climb a shallow hill. The speed decreases to TOTL WEIGHT Capteen! Eyee kant gib yah nooo mur pawar sew stup askeen! THRUST DRG Steep Hill (made by elevator control) 90 o Weight opposite lift Weight o Lift 90 to flight path E Fig mph just as it did in the car. ttempting to climb a steeper hill (irplane C) slows our speed down to 70 mph. We can t climb the hill we just selected faster than 70 mph because we don t have the extra horsepower (thrust) to do so. s we continue to steepen the angle of climb, our airspeed decreases further, just like the car s speed did. Here, however, is where the airplane goes its own way. irplanes need to maintain a minimum forward speed for their wings to produce the lift required to stay airborne. Ever wonder why airplanes need runways? Same reason long jumpers do. irplanes (and long jumpers) must attain a certain speed before they can take flight. This minimum forward speed is called the stall speed of the airplane. It s a very important speed that changes with variations in weight, flap setting, power setting and angle of bank. It also varies among airplanes (no need to worry because

5 Chapter 2 - erodynamics: The Wing is the Thing 5 later I ll show you how to recognize when you re near a stall). s long as the airplane stays above its stall speed, enough lift is produced to counter the airplane s weight and the airplane will fly. If the stall speed of irplane C (Figure 7) is 60 mph, then climbing at a slightly steeper angle will result in insufficient lift for flight. We call this condition a stall. Done unintentionally, it leads to such primitive linguistic sounds as Uh-oh, yipes, ahhhhh, as well as I think I need to have my chakras balanced. Needless to say, these sounds make passengers reluctant to ever fly with you again. This is why some of your time as a student pilot will be spent finding out about stalls, and doing them (intentionally, that is). Instructors have special biological filters installed that keep them from making these sounds on those rare occasions when you unintentionally stall the airplane. That s why they are sometimes referred to as certified flight instructors. What you need to know is that airplanes with a lot of power (like jet fighters) can climb at steep angles; those with limited power, however, must climb at less steep angles. Knowing it s extra thrust and not extra lift from the wings that is responsible for the climb allows you to draw some interesting conclusions. For instance, anything that causes the engine to produce less power prevents you from achieving your maximum rate of climb. mong the things resulting in less power production are high altitudes and high temperatures. More on these factors a bit later. *ngles are exaggerated so you don't need to use too much imagination! C POWER, CLIM NGLE ND IRSPEED Steep Climb ngle Normal Climb ngle Straight & Level t this point you should be asking an important question. I certainly don t mean questions of the Zen- Koan type, such as What is the sound of one cylinder firing? or If an airplane lands hard in the forest and nobody is there to hear it, does it really make a sound? good question for you to ask is, How can I determine the proper size hill for my airplane to climb? Let s find out. irplanes have a specific climb attitude (steepness of hill) that offers the best of all worlds optimum climb Full Power Full Power Full Power Even with full throttle (maximum power), 120 the airplane slows down as it attempts to ascend a steeper hill. Pilots adjust their climb angle (hill size) by selecting an attitude that gives them a specific climb airspeed. Fig. 7 Wow! Those aftermarket add-ons are really something, aren t they? KNOTS 120 KNOTS 120 KNOTS performance while keeping the airplane safely above its stall speed. You can determine the proper climb attitude for your airplane by referring to its airspeed indicator. With climb power applied (usually full throttle in smaller airplanes) the pitch attitude is adjusted until the airspeed indicates one of two commonly used climb speeds. These speeds are known as the best angle of climb and the best rate of climb airspeed. The best angle of climb provides the greatest vertical gain in height per unit of forward travel; the best rate of climb provides the greatest vertical travel per unit of time. You select best angle when you need to get up in the shortest possible distance, usually to clear an obstacle. You choose best rate of climb to gain the most altitude per minute. Let s put this in concrete terms. Say there s a concrete tower 750 feet high half a mile off the end of the runway. You definitely want to be above 750 feet at one-half mile out, and you

6 6 Rod Machado s Private Pilot Handbook What happens if the engine quits? The airplane becomes a glider, not a rock. don t really care how long it takes you to get there. Your choice is best angle of climb. Under normal circumstances you will climb at the best rate of climb speed, or a bit faster. Sometimes pilots climb at airspeeds slightly faster than the two reference airspeeds when better over-the-nose visibility is required. Raising the nose of the airplane results in a slower airspeed; lowering it picks up the pace. Where you place the nose how steep you make the hill determines what happens on the airspeed indicator. Unlike the ground-bound world, pilots decide how steep the hills in the air are going to be (within limits of course!). With just a little experience, you ll be able to determine the correct size hill (nose-up attitude) by simply looking out the front window instead of having to rely solely on the airspeed indicator. When I was a student, any specific airspeed was the one place on the dial where the pointer never went. I was not gifted with much During a climb with full power, where you place the nose how steep you make the hill determines what happens on the airspeed indicator. coordination as a youngster. My reflexes were so slow I was almost run over by two guys pushing a car with a flat tire. I m a living exhibit that one can be a competent pilot even without the coordination and reflexes of a 13-yearold Olympic gymnast. Descents While engine power moves a car uphill, gravity pulls it down. Without your foot on the throttle, the car s downward speed is determined by the steepness of the hill it s descending (Figure 8). The steeper the hill, the faster it goes. If the hill shallows out, then the speed decreases. If the hill becomes too shallow, then some power is necessary to maintain sufficient forward speed. irplanes can also move downhill without power. Just lower the nose (Figure 9) and you ll get what appears to be a free ride (it isn t, but let s not get into that). You can adjust the nose-down angle with the elevator control and descend at any (reasonable) airspeed you desire. You now have the answer to a question I guarantee every firsttime passenger will either ask or want to ask you: What happens if the engine quits? The airplane becomes a glider, not a rock. Unlike climbing, you may elect to descend within a wide range of airspeeds. There are, however, many factors to be considered such as forward visibility, engine cooling and the structural effects of turbulence on the airframe (all of these items will be discussed in this book). However, during the last portion of the landing approach (known as final approach), you should maintain a specific airspeed. Usually this speed is at least 30% above the airplane s stall speed.when preparing to touch down, excess airspeed or erratic control forces often lead to difficulty in making a smooth landing (it s also the reason pilots make good humored fun of one another). Now that The Four Forces are no longer a mystery, let s examine the most important force lift. Without this force, airplanes are nothing more than very expensive ground bound vehicles that are impossible to parallel park. N UTOMOILE'S FOUR FORCES IN DESCENT The steeper the angle of descent, the more the car's total weight (), acts forward (D), in the direction of thrust (E). Drag (F) primarily results from air resistance. Lift from the road () still acts perpendicular to the car's motion and is still equal and opposite to the car's weight on the road (C). DRG F Steep Hill C N2132 D 90 o Weight opposite lift Weight TOTL WEIGHT o Road lift 90 to motion N IRPLNE'S FOUR FORCES IN DESCENT Steep Hill Weight opposite lift Weight TOTL WEIGHT I hope that's a mattress factory at the end of this street! THRUST/ or WEIGHT The steeper the angle of descent, the more the airplane's total weight (), acts forward (D), in the direction of thrust (E). Drag (F) is aerodynamic drag. Lift () still acts perpendicular to the flight path (the relative wind). Lift (), is still equal and opposite to that part of the weight (C) which acts 90 degrees to the flight path. F Fig. 9 DRG C D 90 o o Lift 90 to motion THRUST/ or WEIGHT E E Fig. 8 *s you ll learn shortly, the thrust vector is not lined up with the axis of the airplane because the airplane must always fly at a slight positive angle to its direction of motion.

7 Chapter 2 - erodynamics: The Wing is the Thing 7 The Wing and Its Things Defining the Wing In ground school many years ago, my instructor asked me about the origin and definition of the word wing. I replied, Ma am, I think it s Chinese and means the arm of a bird. She mumbled something about why many animals eat their young at birth, then went to the dictionary to look up the definition. Wing was defined as a moveable, paired appendage for flying. She looked at me and said, Well, what does that sound like to you? I said, Well ma am, that sounds like the arm of a bird to me. We agreed to disagree, even though I was right. The wing has several distinct parts. These are the upper cambered surface, lowered cambered surface, leading edge, trailing edge and chord line (Figure 10). Notice that the upper cambered (meaning curved) surface seems to have a greater curve to it than the lower cambered surface. This isn t accidental. In fact, this is so important that we ll talk about it in detail shortly. Perhaps the only term whose definition isn t intuitively obvious is the chord line. The chord line is an imaginary line connecting the leading edge with the trailing edge. elieve me, there is no line inside the wing that looks like this. It s only imaginary, just like the arrows showing Your author is especially proud of his Cessna lifting body airplane. He believes it has a glide ratio similar to a meteorite. Take special notice of the canvas cooling system! WIND lthough wings vary in size and shape, they all have the same five basic components NGLE OF TTCK The angle of attack in this example is 18 degrees. (exaggerated) Chord line The angle of attack is the angle between the chord line and the relative wind (this is the wind that's blowing on the airplane's wing). Fig. 11 The Four Forces. When the shoe salesman points to your foot and says, Your toe is here, you want to respond by saying, Thanks, I ve been looking for that. In reality he or she is pointing out the position of something not visually obvious. The chord line does something similar. Given the wing s curved surfaces, it s difficult to tell which way the wing points. Since engineers don t like uncertainty, they agreed that the chord line will represent the general shape of a wing. Leading edge THE FIVE COMPONENTS OF WING Upper cambered sur ce Lower Chord line cambered Trailing edge The red line shows the chord line of the wing. How the Wing Works To understand lift, you must visualize how the wing attacks the air. eronautical engineers talk about the wing contacting or attacking the air at a specific angle. This occurs in much the same way a pitbull attacks a mailman mouth first. What part of the wing does the attacking? Is it the leading edge? Is it the trailing edge? Or, is it the bottom of the wing? This is where the definition of chord line becomes useful. ecause wings come in variable sizes and shapes (just like airline pilots), it is sometimes difficult to determine exactly how and where the wind strikes the wing. Fortunately, the chord line substitutes as a general reference for the shape of the wing. If I say that the wind blows onto the wing at an 18 degree angle, I m saying that the angle between the wind and the chord line is 18 degrees (Figure 11). This distinction, although seemingly trite, is as important to an engineer as tightly-stitched pant seams are to a matador. Only one more definition need be absorbed before the secrets of lift fa surface The chord line is an imaginary line connecting the leading edge to the trailing edge of the wing. Fig. 10

8 irplane s Motion 8 Rod Machado s Private Pilot Handbook Fig. 12 are revealed. That term is called the relative wind (which is not a reference to an uncle who s like a Kamikaze pilot the type of person who does all his bragging ahead of time). Relative Wind Movement of an airplane generates wind over the wing. This wind is called the relative wind because it is relative to (or results from) motion. For instance, in Figure 12, no matter which way the jogger runs, he feels wind in his face that s relative (opposite and equal) to his motion. Relative wind is movement-generated wind. To illustrate this point, stick your hand out the window of a moving automobile (keep all other body parts inside, please). You ll feel wind blowing opposite the motion of the car. Drive a car backwards on the freeway and you will feel wind, and hear a lot of horns, blowing from directly behind you (you ll also attract the police, so have those Relative wind is independent of which way the airplane s nose is pointed. doughnuts ready). Relative wind is movementgenerated wind that s equal and opposite to the motion of the airplane. Move the airplane forward as shown by irplane in Figure 13 and wind blows on its nose. Move the airplane up or down a hill and wind still blows on its nose (irplanes and C). Drop an airplane and the wind blows on its belly (irplane D). s far as irplane D is concerned, the wind is blowing on its belly despite the level attitude (as for the passengers, they re probably underneath Relative Wind Relative Wind irplane s Motion N2132 N2132 the front seat in the fetal position, making spiritual transmissions that don t require a radio do not scare your passengers. It isn t nice, and they don t like it). Relative wind blows from a direction that s opposite the direction of airplane motion, irrespective of what direction the airplane s pointed. The following point is so important I want you to put one finger in your ear. Go ahead, do it before reading any further! I want you to do this because I don t want this information to go in one ear and out the other. The important principle to remember is that relative wind is independent of which way the airplane s nose is pointed. Relative wind is opposite in direction and equal to the airplane s velocity. Let s see how the wing actually attacks the wind to develop lift. ttacking the ir Hunting is a sport to some people. It s also a sport where your opponent doesn t know it s a participant. ttacking a mammal means that the hunter must point his weapon precisely at the prey. The hunter looks though the gunsight and sees the path of the bullet. n airplane is unlike a gun (and a car) in that its vertical climb path is different from its RELTIVE WIND DEPENDS ON YOUR MOTION The relative wind is opposite and equal to the motion of the airplane. C The relative wind is opposite and equal to the motion of the airplane. Note: irplane shown at a positive angle of attack. irplane s Motion D Relative Wind N2132 The relative wind is opposite and equal to the motion of the airplane. irplane s Motion Relative Wind The relative wind is opposite & equal to the motion of the airplane. N2132 What's the number for 911? Fig. 13

9 Chapter 2 - erodynamics: The Wing is the Thing 9 ngle of ttack THE NGLE OF TTCK ob, when I say get your nose down, I mean the airplane's nose. irplane's Motion wise man (who is also a poet) says: May the runway rise slowly to meet thee, May your landings make nary a sound, May your Hobbs meter fail completely, May you never hear the words, Go around. Relative Wind N2132 incline (the direction it points upward). Remember that 750 foot tower off the end of the runway? On takeoff, if you point your airplane slightly above the top of that obstacle (like a rifle sight), it s unlikely that you re going to clear it. In fact, the only thing being cleared is the area as the firemen try to talk you down from the side of that tower. Remember, airplanes with limited thrust have shallower climb paths unlike some fighter jets. The most important principle to understand here (put that finger back in the ear) is that the nose (therefore the wing) can be pointed on an incline that s different from the actual climb path. n angle exists 5 Degree ngle of ttack Fig. 14 between the amount the wing is inclined and its climb path (you ll soon see why). Remembering that the relative wind is always equal and opposite to the flight path, it s more precise to say that an angle exists between the chord line and the relative wind. This angle is known as the angle of attack (Figure 14). Figure 15 shows the wing (chord line) of irplane making a 5 degree angle to the relative wind. more common way of saying this is that the wing s angle of attack is 5 degrees. irplanes, C and D show increasing angles of attack of 10 degrees, 30 degrees and 45 degrees, respectively. The greater the difference between the wing and the NGLE OF TTCK relative wind, the greater the angle of attack. nd, as you re about to see, the wing s lift is directly associated with its angle of attack. How Lift Develops The wing is the ultimate air slicer. s powerful as any Ginzu knife, Samurai sword or karate chop, it s a precision device for slicing air in a very specific way. Wings are expressly built to plow through air molecules separating them either above or below while offering little resistance in the horizontal direction. ny horizontal resistance slows the wing down. This horizontal resistance is drag, and it s definitely a case of less being better. 10 Degree ngle of ttack Relative Wind C 30 Degree ngle of ttack *ngles are exaggerated for comparison Relative Wind D 45 Degree ngle of ttack Now, that bright one over there is Jupiter. Relative Wind Relative Wind Fig. 15

10 10 Rod Machado s Private Pilot Handbook Figure 16 shows how the airfoil splits the wind when it s at a 10 degree angle of attack. irflow strikes the leading edge of the wing forcing some air over and some under the airfoil (a fancy name for a wing). oth the air flowing over and the air flowing under the wing are responsible for generating lift. Let s first examine how the airflow striking the bottom of the wing produces some of the total lift that is developed. Impact Versus Pressure Lift Sticking your hand out the window of a moving automobile does two things: it demonstrates how a relatively flat surface develops lift and it signals a left turn. Figure 17 shows that wind is deflected downward when it strikes your hand. ccording to Sir Isaac Newton, who knew about such things, for every action there is an equal and opposite reaction. Wind deflected downward by the airfoil creates an upward (opposite) movement of the wing. This upward movement is caused by the impact energy of billions of tiny air molecules striking the underside of the wing. lso, higher pressure on the bottom surface of the wing results from this molecular impact. The wing moves upward as if it were pushed from below. This type of lift is known as barn door or impact lift. It generally contributes only a small portion of the total lift produced by the wings, which means that man and woman do not fly by barn door lift alone. If we could, it would mean people in the Midwest would report flying barn doors instead of UFOs. more subtle and powerful form of lift occurs from curved airflow over the Wind deflected downward by the airfoil creates an upward (opposite) movement of the wing. top of the wing. ending the Wind With the Wing The Japanese invented the art of paper bending and called it origami. They then experimented with people bending and called it judo. This art was not perfected, however, until the airlines adopted the practice, which is referred to as flying coach. irliners (indeed all airplanes) bend something else they use their wings to bend the wind. Wind bending did not sound sophisticated enough to explain why airplanes fly, so it was given a fancy Greek title. We call wind bending aerodynamics. Simply stated, the wing is a precision device for bending or curving the wind downward. ut how does bending the wind over the wing create lift? Let s find out. Figure 18 shows a cross section of an airfoil. Examine its shape carefully. t small angles of attack, air flowing above the wing is bent or curved with great precision as it follows the upper cambered surface. rather straight surface on the bottom of the wing leaves the air underneath relatively unbent. ending or curving the wind above the wing forces air to travel a greater distance than the straighter airflow below. If the wind above the wing is to reach the trailing edge at nearly the same time as the wind below (science says that the air above the wing actually reaches the trailing edge sooner than the air below the wing), it must speed up to cover the greater distance. For example, assume you are walking your pitbull (named ob) on a leash. You are on the sidewalk and ob is walking in the gutter (Figure 19). ob encounters a parked VW and decides to walk over the car rather than around it (remember, it s a pitbull). Obviously, the distance over the car is greater than the distance you will travel on the sidewalk. In order for ob to avoid being choked by the leash he will have to speed up slightly as he covers this greater distance. irflow striking the hand is deflected downward. This imparts an equal and opposite upward force to the hand. irflow IRFLOW OVER ND UNDER WING Some irflow Wing O Lift from an airfoil is produced by air flowingoverandunderthewing. IMPCT LIFT High pressure is created on the bottom of the hand by impacting air molecules. Goes Ov r Some irflow Goes Under ngle of ttack pproximately 10 IRFLOW ir e Total Lift IRFLOW OVE ND ELOW THE WING (at a small angle of attack) bove Wing M ust T Wing rav el D s G r re te ir elow Wing Travels Less Distance t low angles of attack the air above the wing is curved while the air below the wing is relatively straight. Fig. 18 a i Fig. 16 Fig. 17 t a nc e

11 Chapter 2 - erodynamics: The Wing is the Thing 11 DIFFERENT DISTNCES IN CURVTURE OVE THN ELOW THE CR (WING TOO) Distance covered byl pi bu t l Hey! What re You Doing Up There? This pilot was caught checking for those mysterious little ernoullis that are responsible for the creation of lift. He didn t find any. Fig. 19 Direction of walk Distance covered by master s ob (the pit bull) walks over the car (shaped like a wing) he must cover a greater walking-distance than his master. Therefore, he must speed up to keep from being choked by his master's leash. Did You Know... Did you know that the reduction in air pressure above the wing caused by higher velocity airflow is actually less than that of a young child sucking on a bottle? When you take into account the total wing area, even a small amount of pressure reduction per square inch of wing adds up to a significant total pressure difference. Each square foot of wing area on a Cessna 172 provides 13.2 pounds of lift. Since there are 144 square inches in a square foot, you can see that each square inch of wing provides less than.1 pound of lift. Idea Source: arry Schiff, The Proficient Pilot Do you notice the resemblance of the VW s profile to a wing? It s curved on top and rather straight on the bottom. s air flows over the wing, it curves and speeds up. Something remarkable happens when air, flowing over a surface, increases its speed. physicist named ernoulli (pronounced RR- NEW-LEE) figured out that the faster air flows over a surface, the less pressure it exerts on that surface. High velocity airflow over the wing causes a slight decrease in pressure on the wing s upper surface. In other words, the pressure on top of the wing is now less than the pressure on the bottom of the wing (Don t ask why. It has to do with translational kinetic energy and explaining that will give you something that feels like a two scoop lobotomy). Known as ernoulli s principle, this wonderful trick is Same Pressure Same Pressure ERNOULLI'S PRINCIPLE what keeps airplanes from being large and expensive doorstops. Try an experiment. Take a piece of writing paper and hold it in such a way that the top surface is curved downward (Figure 20). With a little imagination, you can see how the bent paper is similar in appearance to the wing s upper cambered surface. low over the top of the paper. What happens? The paper should rise upward (a hundred years ago the Lower Pressure Higher Pressure Fig. 20 lowing over the top of the paper creates higher velocity airflow which reduces pressure causing the paper to lift upward similar to a wing's lift.

12 12 Rod Machado s Private Pilot Handbook Different Designs Indians would have made you chief of the tribe for this magic demonstration!). Increasing the velocity of air over a surface reduces the atmospheric pressure on that surface. This means you have low pressure above the wing and high pressure underneath. Since high pressure always moves toward low pressure, the wing (which just happens to be in the way) is pushed upward in the process. ernoulli s principle is responsible for the majority of lift developed by the wings of most small airplanes. Impact or barn door lift is an accessory, but ernoulli does most of the work. Most wings are designed with their upper surface curved and their lower surface relatively straight. ecause of the wing s shape, even at a very small angle of attack, a cambered wing still adds a slight curve and acceleration to the wind. This produces the lift you learn to love, particularly if you think an airplane should fly. There are more wing shapes than there are body shapes at a Weight Watchers convention. Each shape has its own lift and drag characteristics (bodies too!). Figure 21 shows a variety of these different airfoils. ig ones, thick ones, small ones, skinny ones. They all serve one purpose generating lift. Sometimes, however, the engineered shape of the wing all by itself can t produce the necessary lift for flight. In these instances the angle of attack becomes the major player in lift production. ngle of ttack nd the Generation of Lift During takeoff on a commercial airliner, have you ever noticed that the pilot always raises the nose slightly to begin the climb after attaining a minimum forward speed? This is called rotation and it isn t something done to the airplane s tires. s the airplane accelerates for takeoff, it eventually reaches a sufficient speed to begin flying. t this relatively slow n intimate and sizzling relationship exists between speed, however, the wing s engineered curve isn t capable of and lift. angle of attack curving or deflecting enough air downward to produce the necessary lift for flight. This is why the airplane doesn t hop off the ground like a grasshopper that just landed on a hot barbecue. The pilot must do something extra to add an additional curve to the wind. Raising the nose slightly increases the angle of attack. This forces the air to undergo an additional curve greater than what the engineered shape of the airfoil can produce. Figure 22 depicts this process. With this additional curvature, air travels a greater distance, its speed increases, pressure lowers on top of the airfoil, and sufficient lift to begin flying is produced at a slower airspeed (thanks for the lift, ernoulli!). Greater impact lift results from increased exposure of the wing s lower surface to the relative wind. The result is that an increasing angle of attack permits the airplane to produce the necessary lift for flight at a slower airspeed. Fig. 21 DIFFERENT IRFOILS irfoils are a lot like people: Some, are just average - Others are sleek & thin - Some are different - nd others are built to hold couches down Duhhh! Help! My P is 300/190 and that s PSI. Who are you looking at man? I m 370, and that s big when you consider that 360 is a full circle!

13 Chapter 2 - erodynamics: The Wing is the Thing 13 Now you know how airfoils generate the required lift at slower airspeeds. You also know why airplanes, taking off or landing at slower speeds, seem to have a rather nosehigh attitude. ut what happens at higher airspeeds? Have you noticed that in cruise flight, at cruise airspeeds, airplanes fly at near-level flight attitudes? Figure 23 shows an airplane at several different angles of attack. t higher speeds airplanes can fly at lower angles of attack because the wing s shape generates sufficient lift. Slow the airplane and the wing must artificially bend the wind by increasing its angle of attack. n intimate and sizzling relationship exists between angle of attack and lift. If lift and angle of attack were Rhett and Scarlet, tlanta wouldn t be the only thing on fire. t small angles of attack (such as during cruise flight), the engineered shape of the airfoil generates sufficient lift for flight as long the airspeed is high. The impact of air underneath the wing doesn t play as big a role in lift development at higher (cruise) speeds because less of the wing s underside is exposed to the wind. In summary, the slower an airplane moves, the greater the angle of attack needed for flight. There is, however, such a thing as too much of TWO FORMS OF LIFT 1 Lift From Low Pressure t large angles of attack the airflow is forced to curve beyond the engineered shape of the wing. 2 Impact Lift Impact lift on the bottom of the wing increases at a high angle of attack. More air is deflected downward which results in an equal and opposite upward movement of the wing. It didn't seem to look quite this way in those da Vinci parchments. Fig. 22 a good thing. end the air too much and instead of flowing smoothly over the wing and creating lift, it bubbles and burbles and pretty much fails to be very uplifting. We call this condition a stall. What kind of instructor would I be if I didn t say, Let s find out how and why a wing stalls? POWER TO SPRE RELTIONSHIP ETWEEN NGLE OF TTCK & SPEED 150 Knots 0 3 ngle of ttack 100 Knots 0 8 ngle of ttack 80 Knots 0 12 ngle of ttack 60 Knots 0 18 ngle of ttack If you own one of these little babies, drag isn t your problem fuel is. With speed variations in level flight, the relationship between the angle of attack and airspeed is clearly shown. With increasing airspeed, the airplane requires a smaller angle of attack to remain airborne. s the airplane's airspeed decreases, a larger angle of attack is necessary. Fig. 23

14 14 Rod Machado s Private Pilot Handbook STLLS Stall, ngle of ttack, nd How the Nose Knows pilot s job is to work The Four Forces, maintain lift, and avoid the burbling air condition that results in a stall. Think of air molecules as little race cars moving over the wing (Figure 24). Each car and air molecule has one objective: follow the curve over the wing s upper cambered surface. Of course, if the wing is at a low angle of attack, the curve is not very sharp and it s a pretty easy trip (Figure 24). ut look at the curve made by these cars and air molecules when the wing is attacking the wind at a very large angle. s the angle of attack exceeds approximately 18 degrees (known as the critical angle of attack for reasons you will soon see), these speed-racer air molecules don t negotiate the turn (Figure 24). When this happens, they spin off or burble into the free air, no longer providing a uniform, high velocity, laminar airflow over the wing (Figure 25). The wing stalls. (See Seeing the Stall in Postflight riefing #2-6, page 46 for a view of how the wing stalls.) Remember, according to ernoulli, lower velocity airflow over the wing produces less lift. There is still impact lift provided by air molecules striking the underside of the wing, but we ve already learned that this doesn t provide nearly enough lift to sustain the airplane. When there s less lift than weight, bad things happen to good airplanes. The wing goes on strike and stalls. bandoned by ernoulli, gravity summons the airplane to earth on its own terms. ll wings have a critical angle of attack (the angle varies slightly among airplanes). eyond this angle the wing and the wind don t work and play well together. ll the whispered theory in your heart won t overcome the laws of physics and aerodynamics. The wing police are always watching. Exceed the critical angle of attack and the air THE RDICL INSTRUCTOR Some instructors dream of letting their students experience a stall from a very unusual vantage point. (Just kidding!) THE CRITICL NGLE OF TTCK ir molecules, like race cars, can easily curve around a wing at a small angle of attack. WING LOW NGLE OF TTCK When forced to undergo too large a curve, air molecules, like race cars, may skid off the top of the wing resulting in a stall. WING CRITICL NGLE OF TTCK Don't worry, it's a rental. Fig. 24 STLLED VS. UNSTLLED WINGS When the wings exceed their critical angle of attack, airflow over their upper surface, becomes chaotic and starts to burble. It is no longer smooth, high velocity airflow. Consequently, lift decreases. WINGS STLLED ngle of ttack Wings operating below their critical angle of attack allow smooth, high velocity airflow to move over their upper surface. This keeps the pressure on the wing's upper surface low and maintains the required lift. WINGS NOT STLLED Fig. 25 ngle of ttack

15 Chapter 2 - erodynamics: The Wing is the Thing 15 Pilot raises the nose too steep during a climb. The critical angle of attack is exceeded & the airplane stalls. STLLING ND EXCEEDING THE CRITICL NGLE OF TTCK The pilot lowers the nose (adds full power if not already so) and reduces the angle of attack (to less than the critical value). This action reestablishes smooth airflow over the top of the wing. Once the airplane is no longer stalled, the pilot raises the nose slowly. I think I ve just reached my target heart rate for the day! C Fig. 26 Pilot now resumes climb without exceeding the critical angle of attack. D Now my blood pressure is 3 over 1! molecules won t give you a lift. Sounds serious, and it can be. Fortunately, there s a readily available solution, and it is not screaming, Here, you take it to the instructor. Put your finger back in your ear; here comes the important This airplane is stalled with its nose pointed downward while doing 150 knots. Fig. 27 STLL RECOVERY /C MOTION The only way to recover is for the pilot to move the elevator control forward. s contrary as this may seem, lowering the nose slightly by moving the elevator control forward (or releasing the back pressure on the elevator control) reduces the angle of attack to less than its critical value. The wing will once again start flying and the airplane will no longer be stalled. t this point the pilot should pull back slowly without (hopefully!) exceeding the critical angle of attack. This airplane is moving horizontally at 100 knots and stalls because the pilot pulled too hard on the elevator control. ob, this is no time to be lighting candles! /C MOTION Recovery from the stall is accomplished by moving the elevator control forward (or releasing back pressure) and decreasing the angle of attack to less than its critical value. These are illustrations for demonstration only. It's unlikely you'll ever find your airplane in these attitudes at these airspeeds. stuff again. You can unstall a wing by reducing the angle of attack. You do this by gently lowering the nose of the airplane with the elevator control (Figure 26, sequence,, C and D). Easy does it here, Power Ranger. Once the angle of attack is less than its critical angle, the air molecules flow smoothly over the top of the wing and production of lift resumes. It s as simple as that. Please don t ever forget this. Why am I making such a big deal out of this? Why did I make you put your finger back in your ear? ecause in a moment of stress (having the wing stop flying creates stress for many pilots) you will be inclined to do exactly the opposite of what will help. It s a natural pilot response to pull or push on the elevator control to change the airplane s pitch attitude. During a stall, as the airplane pitches downward, your untrained instinct is to pull back on the elevator control. You may yank that critter back into your lap, and the result will not be good. The wing will remain stalled and you, my friend, will have the look of a just gelded bull. If the wing stalls, you need to do one very important thing: reduce the angle of attack to less than its critical value. Only then does the wing begin flying again. dding full power also helps in the recovery process by accelerating the airplane. The increase in forward speed provided by power also helps reduce the angle of attack. Don t just sit there with stalled wings. There s a reason why you are called the pilot-in-command. Do something. ut do the right thing. Stall at ny ttitude Or irspeed You should realize that airplanes can be stalled at any attitude or at any airspeed. Put your finger back in your ear. It makes no difference whether the nose is pointed up or down or whether you are traveling at 60 or 160 knots. Whether an airplane exceeds its critical angle of attack is independent of attitude or airspeed. Figure 27 depicts two instances showing how this might happen. irplanes have inertia, meaning they want to keep on moving in the direction they are traveling. irplane is pointed nose down, diving at 150 knots (don t try this at home!). The pilot pulled back too aggressively, forcing the wings to exceed their critical angle of attack. The airplane stalls. Wow! Imagine that. It stalls nose down at 150 knots! What must the pilot do to recover? The first step is to decrease the angle of attack by moving the elevator control forward or releasing back pressure on the control wheel/stick (remember, pulling back on the elevator control was probably responsible for the large angle of attack inducing the stall in the first place.) The airplane will always stall when the wings exceed their critical angle of attack. It s the law! This re-establishes the smooth, high velocity flow or air over the wings. The airplane is once again flying.

16 16 Rod Machado s Private Pilot Handbook The second step requires applying all available power (if necessary) to accelerate the airplane and help reduce the angle of attack Once the airplane is no longer stalled, it should be put back in the desired attitude while making sure you don t stall again. Stalling after you ve just recovered from a previous stall is known as a secondary stall. Unlike secondary school, it is not considered a step up, especially by the participating flight instructor. (You ll know your instructor is unhappy when you hear her make subtle statements like, Hmm, come to think of it, childbirth wasn t all that painful. ) Stalling an airplane intentionally, at a safe altitude, is actually fun, or at least educational. Stalls are relatively gentle maneuvers in most airplanes. Stalling an airplane close to the ground, however, is serious business because it is usually not an intentional act. During flight training you ll have ample practice in stall recovery. Managing a stalled airplane is one thing; managing your natural instincts, another. For example, a typical stall trap you could (literally) fall into involves a high sink rate during landing. While on approach, you might apply back pressure on the elevator, attempting to shallow the descent. If you exceed the critical angle of attack the airplane will stall. The runway now expands in your windshield like a low orbit view of a supernova. If you follow your untrained instincts and continue to pull backward on the elevator, the stall deepens. Trained pilots know better. They are aware of the possibility of stalling and apply the appropriate combination of elevator back pressure and power during landing to change the airplane s glide path without exceeding the critical angle of Managing a stalled airplane is one thing; managing your natural instincts, another. Hey! What did you do to my controls? attack (your instructor will show you the appropriate use of elevator and power during landing). How do pilots know the proper amount of rearward movement to apply to the elevator? How do they know they won t stall the airplane? If there was an angle of attack indicator in your airplane, stall recognition would be easy. You d simply keep the angle of attack less than what s critical for that wing. ngle of attack indicators, although valuable, are very rare in small airplanes. Fortunately, there are several ways to recognize a stall s warning signs. Five Stall Warning Signs There are five good early warning clues to indicate the onset of a stall. Good pilots know and watch for all of them. First, an unmistakable buffet or shaking is usually felt in the airplane and on the flight controls. You might think your instructor is shaking the other set of controls trying to get you to let go of them. Not true. s airflow begins to separate from the top of the wing, it buffets (burbles). For those with only food on their mind, this is UFF-ETTES, not OO-FYS. One means getting lunch, the other can mean losing it. Inside the cockpit, ad Thing I recall a student who was a quick study and was a real pleasure to teach. During his first introduction to stalls he elected to pull all the way back on the elevator control and hold it there. He couldn t talk. He just looked at me, pointed to the elevator control with his right hand and said, D THING. ecause of this, the airplane remained in a stalled condition. When stalled, the flight controls are relatively ineffective. s one wing dipped, he turned the wheel, attempting to raise the paralyzed half of the plane. Discovering that his controls were ineffective, he reached over and grabbed mine (I just love it when that happens!). He was thinking I might have played a dirty trick by somehow disabling his set of flight controls. Upon discovering that my controls didn t work either, he grabbed my arm and made a sound somewhat similar to the stall horn but higher in pitch (probably causing bats to fly into walls). Once he let go of the controls, the airplane nosed forward on its own (as it s designed to do), decreased its angle of attack and recovered from the stall all by itself. Had he intentionally lowered the nose in the first place, I wouldn t have had an enormous bruise on my arm. this buffet is felt as vibration and is one of the best early warning clues to an impending stall. If you feel this buffet, simply release some of the backward pressure on the elevator until the buffeting stops. Second, flight control response usually diminishes when the airplane approaches a stall. If you feel the controls becoming mushy or less effective, this is your cue that things aren t going well. Think stall. Think lower angle of attack. Think quickly. Third, when the airspeed indicator is nearing the beginning of the white or green arc, you re approaching stall territory. irplanes must have airspeed to fly. Slow the airplane down too much and it will eventually stall. Keep an eye on the airspeed indicator and make sure it s a healthy distance above the beginning of the green arc. This is the power-off stalling speed without flaps and gear extended (called the clean configuration). When approaching to land, the usual recommendation is to keep the airspeed at least 30% above stall speed. This gives you room to accommodate slight, unexpected variations in airspeed caused by poor technique or wind shear. good visual clue indicating slow speed is when cars on the road below are moving faster than you are... at rush hour.

17 Chapter 2 - erodynamics: The Wing is the Thing 17 Having a bird fly up your exhaust stack is a good sign that you re flying too slow in the traffic pattern. Fourth, a distinct difference in sound occurs when approaching a stall. When I was taking lessons this sound usually came from my instructor yelling, Lower the nose, goofball, lower the nose! When airflow strikes the airplane at higher angles of attack, it often makes a different, but recognizable sound. Fifth, all modern airplanes have stall warning horns or lights. These tattletale devices activate a few knots above stall speed and provide an excellent pre-stall warning when they re working and when the pilot s listening. On some airplanes it s difficult to test the stall warning horn during the preflight walkaround. That means you won t know it s not working until you need it. You will also be surprised what you can ignore while concentrating on the radio, your instructor, or the pretty colors on all the gauges. That s why you must use all the clues, not just one or two. side from these five basic stall warning signs, the seat of your pants provides an additional clue. Let me explain. Stalling Speed, Gee Whiz nd G-Force Ever watched somebody use a personal computer when something goes wrong? What s the first thing they do? They repeat what they just did, as if the electrons might go with a different flow the second or third time around. Computers are highly predictable, and so are airplanes. For instance, when the weight of an airplane increases, the aircraft will stall at higher speeds. That s right, every time. Son of a gun. Let s assume that a lightly loaded airplane begins flying at 42 knots when the angle of attack is slightly less than its critical value (Figure 28, irplane ). t 42 knots, just enough lift is developed to equal the airplane s weight (although the airplane is on the verge of a stall). n increase in weight means the wings must develop more lift to remain airborne. Certainly the airplane won t fly at or beyond its critical angle of attack (18 degrees for this airplane) since this is the built-in design limit of the wing. irplane stalls at the same critical angle of attack, regardless of its weight. 18 o 42 Knots C..S. Under these conditions, the only way a heavier airplane can develop more lift is to move forward at a slightly faster speed. heavier airplane in our example might need a minimum forward speed of 48 knots before the wings develop enough lift to equal the increase in weight as shown by irplane. The critical angle of attack at which the wing stalls never changes, but the minimum speed at which the airplane begins flying does change. If the airplane is heavier, it stalls at a higher speed. The converse is also true; lighter weights mean slower stall speeds. (I don t want to give you the impression that we start climbing during the takeoff once we re at or slightly above the airplane stall speed. That s not true! We normally HOW WEIGHT FFECTS STLL SPEED LIFT WEIGHT This irplane Weighs 1,300 Pounds irplane, weighing only 1300 pounds, starts flying at when it's slightly above 42 knots and the wing is just a little below its critical angle of attack. nother way of saying this is the airplane's stalling speed is 42 knots at this weight. irplane stalls at the same critical angle of attack, regardless of its weight. 18 o 48 Knots C..S. LIFT WEIGHT N2132 N91911 Fig. 28 This irplane Weighs 1,670 Pounds heavier irplane needs to develop more lift than its lighter cousin before it can fly. t a little less than its critical angle of attack, irplane needs a little more than 48 knots of airflow over its wings before lift equals weight. In other words, the stalling speed of the heavier irplane is 48 knots. Despite weight differences between irplanes and, both airplanes stall at the same critical angle of attack.

18 Gravity Gravity Gravity 18 Rod Machado s Private Pilot Handbook HOW CENTRIFUGL FORCE CUSES YOU ND THE IRPLNE TO FEEL HEVIER IN TURN C During a sharp turn on a level road, you feel a newly generated force known as "centrifugal force." This force pushes you out the door while gravity pulls you downward. If the door pops open, you'll move in the direction of the resultant force. Turning on a banked road causes the resultant force to pull you straight down in your seat. The sharper the turn in the car, the greater the resultant force or G-force you feel in your seat (i.e., the heavier you feel). I think some of our vectors fell out of the baggage compartment! irplanes in a bank feel this same increase in G-force (load factor). Remember, if you feel heavier in a turn, then the airplane also feels heavier. Therefore the airplane must increase its lift to remain aloft in a turn. Centrifugal Force MEFLI2 Centrifugal Force MEFLI2 Centrifugal Force Resultant Force Resultant Force Resultant Force Fig. 29 accelerate to a safe margin above stall speed 30% or more depending on what your flight instructor recommends then climb.) Here s the question that wins a free pizza: Is it possible to make the airplane think it weighs more without adding any weight to it? May I have the envelope, please? The answer is yes. (Mushroom, no anchovies, thank you.) Gee whiz, you re probably saying. No, not gee whiz, or Cheese Whiz, but simply G. Think back to the last time you were on a rollercoaster. On the straightaway all you felt was speed. Turns, however, forced you down in the seat. You experienced an increase in your apparent weight when turning because of something known as centrifugal force. This is the same LOD FCTOR G-Force LOD FCTOR CHRT o 60 ank Produces a Load Factor of "2" or "2Gs." NK NGLE in DEGREES Fig. 30 force that makes you feel you might fall out of the car while turning if your seat belt isn t fastened (Figure 29). ecause airplanes bank (like cars on a banked road), centrifugal force and gravity pull you down in your seat. You and the airplane can expect to feel this apparent increase in weight during a turn. The wings don t know what s causing the airplane to feel heavier, and frankly, my dear, they don t give a darn. They only know that it s getting heavier. The steeper the turn, the greater the centrifugal force and the more you and the airplane appear to become heavier. This force is often called G-force. Perhaps it was so named because students often say, Geeeeeeeeeeee whenever they feel In a 60 degree bank at a constant altitude, the airplane experiences 2Gs. 10 lb. weight produces a 20 lb. scale deflection. their apparent weight increase. The term the pocket protector set (or engineers) use for G-force is load factor (we ll use both terms synonymously). Figure 30 shows a load factor vs. bank chart. Using this chart we can assess the exact amount of G-force you ll experience in any given bank while maintaining altitude. ccording to this chart, a 60 degree bank produces 2Gs. This means you and the airplane feel twice as heavy as you actually are. If the airplane weighed 2,300 pounds, its structure is required to support 4,600 pounds in a 60 degree bank at a constant altitude. Here is the most important point about increasing the G-force (finger in the ear again!). If the airplane feels twice as heavy as it actually is, then the lift must double if the airplane is to maintain altitude. How can you increase lift? Creating a larger angle of attack by applying elevator back pressure, going faster by increasing the power, or a combination of both will increase the airplane s lift production. During steep turns you typically apply back pressure on the elevator control while attempting to maintain your altitude. This increases your angle of attack and increases lift to compensate for the increasing G force (Figure 31). Prior to entering a slight bank at an airspeed of 100

19 Chapter 2 - erodynamics: The Wing is the Thing 19 knots, this airplane s angle of attack was 4 degrees. With a slight bank, back pressure is required and the angle of attack increases to 6 degrees as shown by irplane. The increased angle of attack provides an increase in lift but it also exposes more of the wing s underside to the airflow and increases drag. Now the airspeed drops to 93 knots. s the angle of bank steepens the angle of attack must be increased to maintain altitude. The airspeed continues to decrease because of the increased drag. irplanes, C and D depict this sequence. If you were paying attention back in the stall department, you can see where this flight is going. When the bank is very steep, the airplane reaches its critical angle of attack as shown by irplane D. Having slowed to 84 knots, irplane D is now at its critical angle of attack. ny further attempt to increase the bank while holding altitude stalls the airplane. While this particular airplane stalled at 60 knots in level flight, it now stalls at 84 knots in a very steep bank. The moral of this story is that an increase in weight (apparent or real) causes the speed at which the airplane stalls to increase. In other PERCENT INCRESE IN STLL SPEED There are five good early warning clues to indicate the onset of a stall. Good pilots know and watch for all of them. STLL SPEED ND NK NGLE CHRT ank Increases 140 Stall Speed 120 y 40% o NK NGLE in DEGREES SLIGHT NK 6 ngle of ttack 93 Knots 1.1Gs 1.1Gs LRGER NK O 10 ngle of ttack 90 Knots 1.4Gs words, when you feel an increase in G-force (load factor), the airplane s stalling speed is also increasing. Normally you would increase your power in a steep turn to prevent the airspeed from getting too slow. Giving this particular airplane full throttle in a very steep turn might keep the airspeed at 90 knots. This gives you a 6 knot buffer above the steep turn stalling speed in this example. Of course, this assumes that your engine is capable of producing this extra power in the first place. This simply isn t an option in our smaller, horsepower anemic airplanes, is it? 1.4Gs STEEPER NK VERY STEEP NK 14 ngle of ttack 18 ngle of ttack 87 Knots 84 Knots O O O C D 6 O NGLE OF NK ND NGLE OF TTCK RELTIONSHIP 10 O 1.8Gs 14 O 1.8Gs 2Gs 18 O 2Gs s the angle of bank is increased in level flight, the angle of attack must increase to maintain the necessary lift. s the angle of attack increases so does the lift and the drag which causes the airspeed to decrease. Eventually the angle of attack required for level flight reaches its critical value (irplane D). further increase in angle of attack will cause the airplane to stall at a very high speed (also known as an ). The increase in stall speed with bank angle is as predictable as a politician s addiction to podiums. Figure 32 shows that a 60 degree bank increases stall speed by 40%. This is certainly nice information to know, but are you expected to carry a Cray supercomputer and punch in percentages prior to any turn? Of course not. What you need to have is a high index of suspicion. If the seat of your pants says you re weighing in at a level that qualifies you for the main event on a heavyweight boxing card, you should think (quickly) about how the stall speed increases in a turn. For those of you frightened by higher math (adds, take- accelerated stall Fig. 31 aways, times and goes-intos) there is an easier way to calculate this. Figure 33 is typical of the stall charts found in most owners manuals. It provides you with the stall speeds for specific angles-of-bank under variable flap conditions (we ll discuss flaps in a bit). voiding stalls while in a steep bank with smaller, power-limited airplanes means you must be prepared to do two things. t the first sign of a stall, you must unload the wings by releasing back pressure on the elevator and simultaneously reducing the angle of bank. This decreases the load factor and reduces the stall speed. For example, assume that you re in the traffic pattern and are making a turn onto final approach. ecause of poor planning (it happens to everyone once in a while), you find yourself Power Off ngle STLL SPEED CHRT Stalling Speeds Gross Weight 2,550 lbs. Condition Flaps Up Flaps 0 10 Flaps 0 30 Fig. 32 Fig. 33 Of ank Knots =IS

20 20 Rod Machado s Private Pilot Handbook overshooting the runway centerline. Increasing the bank is a natural response to prevent overshooting but it s also a risky one. When the bank increases, the nose wants to lower or pitch down (I ll talk about why this happens later). Pilots typically pull back on the elevator control to maintain altitude in response to a dropping nose. Pulling back on the elevator increases the angle of attack and slows the airspeed. Now the airplane is closer to its critical angle of attack at a lower airspeed. If the airplane was flying slow to begin with, the airplane may stall. What s the solution? Whenever you are making a turn, especially when close to the ground, be especially sensitive to the amount of bank you use. e sensitive to the G- force you experience. If you feel your apparent weight increasing, you now know your stall speed is also increasing. Your derriere becomes the ultimate stall sensing device (and to think you ve been packing that thing around all these years and didn t realize its usefulness). This is what is called, flying by the seat of your pants. If anyone accuses you of sitting down on the job, you just tell them you re testing your stall detector. y the way, there is a time when we want our wings to stall before they experience too many Gs. The speed at which this occurs is called UPWRD DOWNWRD s unusual as it may look, spins are perfectly safe maneuvers to practice (providing you have proper instruction and an airplane certified for Outside wing: spins). For an airplane to spin, it must more lift & first be stalled. When the stall occurs, less drag (not as stalled) rudder is applied in the direction you desire to spin. The elevator is held back and the airplane commences to autorotate. The out- side wing is not as stalled as the inside wing, causing the airplane to rotate about the spin axis. fter the first two turns the airplane's inertial forces balance and it settles into a predictable pattern of rotation. s unusual as it seems, the airspeed stabilizes at 65 to 75 knots for small airplanes while the descent rate can exceed 7,000 feet per minute. Recovery is accomplished by reducing power (if you haven't already done so), neutralizing the ailerons and applying opposite rudder. The elevator is then moved forward to decrease the angle of attack. See Postflight riefing #2-1 for more info on spins. DEFLECTING N DVERSRY The martial arts master gently deflects his opponent's energy in the vertical (up or down) direction. The master feels little horizontal resistance but his assailant feels an unusual amount of discomfort. I know karate & 7 other Oriental words. Fig. 34 Fig. 35 the design maneuvering speed. (See Postflight riefing #2-5). DRG What a Drag You can t get something for nothing in aviation. lthough urt Rutan (a famed airplane designer) comes close, there is no free lunch when it comes time for an airfoil to pay for the lift it develops. Drag is that price. Drag is the natural response to an object s movement through the air. Try an experiment. Stick your head out a moving automobile (amazing THE SPIN ob, do you think this is a good way to enter the traffic pattern? Inside wing: more drag & less lift (fully stalled) No. Spin axis Motion THE WING & THE MSTER Think of the wing doing the same thing as the martial arts master. The wing deflects the air molecules vertically (up or down), attempting to produce as little horizontal resistance as possible. = ir Molecule I am "Wing Man." what you can learn about flying while sticking parts of you out a car window). t 60 MPH your mouth puffs open, your eyes bug out, your head is thrown back and your hair looks like it was blow dried with a Pratt & Whitney jet engine. You look like a rock star! This is all the result of the wind resistance known as drag. Horizontal and Vertical Movement of ir n ikido martial arts master is trained to deflect his adversary vertically without offering any resistance in the horizontal direction. Upon encountering a head-on adversary, the ikido master makes the ruce Lee karate call sound and gently deflects the assailant either upward or downward (Figure 34). Wings do something very similar. They are designed, as you will recall from our earlier discussion, to deflect air vertically while offering very little horizontal resistance (Figure 35). Regardless of how efficient the wing is, some horizontal resistance to forward motion is always present. We discussed drag briefly at the beginning of this chapter, but let s take a somewhat closer look at the two kinds of speed retardant parasite drag and induced drag.

21 Chapter 2 - erodynamics: The Wing is the Thing 21 Drag is a very complicated subject. I ve seen both blood and entire pots of coffee spilled over the lurid details of a subject so arcane that even engineers with multiple degrees disagree. If you want some ammunition for the next session of hangar flying or want more indepth information, see Postflight riefing #2-2 at the end of the chapter. For the moment, what you need to know is that there are two kinds of drag. Parasite drag is the result of friction. Trying to move something through the air results in friction with air molecules. This resistance isn t useful and that s why it s called parasite drag. It s there because it s there. Hang on to the idea that as airspeed doubles, parasite drag quadruples, and there s very little you can do about it. Rats! Induced drag is resistance to motion induced by the wing turning some of its lift into drag. (See Postflight riefing #2-2 if you want a more detailed understanding of induced drag.) What you need to know here is that induced drag does the complete opposite of parasite drag it increases as the airplane slows down. Total Drag and Your Go Far Speed What does all this talk about parasite and induced drag mean to you as For Training Purposes Only a pilot? It could mean as little as knowing how to save a few bucks on fuel, or it could mean as much as knowing how to protect your remarkably fragile hide in the event of an emergency. I gather I now have your full attention again? s the airplane speeds up, induced drag decreases while parasite drag increases. Hmm. Is there a specific speed, a number better than all other numbers, where the total drag on the airplane is at a minimum? Yea verily, there is. Figure 36 shows the airplane s total drag curve, affirming that unlike igfoot, the Loch Ness Squid and Elvis a minimum drag speed actually exists. When the induced and parasite drag curves are added together to produce a total drag curve, there is always a point where the sum of both curves is at a minimum. The lowest spot in the total drag curve is your magic number, a specific airspeed known as the best L/D speed (pronounced best L over D speed, which stands for lift over drag ). Put your finger back in your ear again, this is important! Since minimum drag occurs at this speed, minimum thrust for forward flight must also occur at this speed. So it is written. Verily, so it will be! Wow, this is good stuff! The Cessna 210 owner s manual shows how the best glide speed (not the glide distance) varies with weight. Fig. 37 Low - TOTL DRG - High Slow Induced Drag TOTL DRG Total Drag Fast Parasite Drag Slow - IRSPEED - Fast Parasite drag increases with increasing airspeed while induced drag decreases with increasing airspeed. The total drag curve is a summation of both types of drag. The lowest point on the total drag curve is the point where the airplane experiences the least amount of drag. Fig. 36 Minimum thrust means we obtain maximum forward distance for a given amount of fuel consumption. The best L/D speed yields the maximum range for a tank (or a gallon or a thimbleful) of fuel in a no-wind condition. If L s and D s and minimum thrust don t stick well, try thinking of this magic number as your maximum range or go far speed. (See Postflight riefing #2-3 for a more detailed discussion of the airplane s maximum endurance and maximum range capabilities.) This speed also yields the airplane s maximum power-off glide range, which is where the saving your hide part comes in. Power-off glide is a nice way of saying the engine isn t working at the moment and you are now the pilot of a glider, looking for a landing spot. So immediately establish the proper gliding attitude and airspeed. This is a time when you will want the ability to go as far as is necessary to find that spot. Figure 37 shows an excerpt from the Cessna 210 s Pilot Operating Handbook (POH). This is the maximum distance power-off glide speed for this airplane. This speed should be as familiar to you as sandpaper is to a safecracker. Know the number for your airplane.

22 22 Rod Machado s Private Pilot Handbook If you try to glide at other than the best glide speed, (the speed for best L/D), the airplane won't cover the maximum distance it's capable of traveling. This particular airplane's best glide speed is 65 knots. FLYING OVE OR ELOW EST GLIDE SPEED Fig :1 Glide Ratio at Speed for est L/D (65 Kts.) Less Than 14:1 at Other Speeds No Wind Not to Scale Good Speed Too Fast Too Slow KNOTS KNOTS Stretching the Glide, Saving the Hide friend of mine was reading the newspaper US Today (he s into primary colors). He said, Hey Rod, it says here on page 13 that 52% of the merican public is mathematically illiterate. I said, That s not a good sign. He replied, Yeah, imagine that, nearly one-third of the population can t do math! s a precaution, I have avoided, to the extent possible, using mathematics in this book. However, I want you to think of L/D as a ratio of lift to drag. Don t think about a ratio as a goes-into or division problem. Think of it as comparing one value No, no no. I said I wanted you to put winglets on my airplane! (lift) placed on top of another (drag), separated by a line. power-off glide happens under one of two conditions. One is instructor-induced. It comes accompanied by some ominous-sounding incantation such as You ve just had an engine failure, and the instructor pulling the throttle back to idle. This is a not-really-power-off glide. You may, at this awkward time, be asked what the plane s best L/D speed is, or what the glide ratio is. The other occasion is a real poweroff glide, caused by a real engine failure. This is the time when you wish you were wearing your Kevlar power suit and accompanying roll bar. t moments like that, you really do want to know the best L/D speed and have some idea of what the plane s glide ratio is. In a power-off glide, the best L/D speed allows the airplane to glide a maximum forward distance with a minimum amount of altitude loss. For example, the D20 Katana has a lift-over-drag ratio of approximately 14 to 1 (also written as 14:1) at its best L/D speed (65 kts.). t this speed the airplane experiences minimum drag (at the bottom of the total drag curve) and moves 14 feet forward for every foot it descends. No matter what you do to the airplane, this is the best glide ratio it s capable of attaining. You can rock back-andforth in your seat and yell, Come on big fellow, come on, yee-hawww! and the airplane still isn t going to glide better than 14 feet forward for every foot it drops. If you attempt to glide at some speed other than this one, your airplane simply won t glide as far (Figure 38). It s possible that you re sitting there munching Doritos wondering, Hey, what happens to the glide range if the airplane s weight changes? If so, see Postflight riefing #2-4 for more details on this interesting subject.

23 Chapter 2 - erodynamics: The Wing is the Thing 23 Winglets are another method used to increase wing efficiency. WINGTIP VORTEX ir sneaks from underneath the wing where the pressure is high, to the top of the wing, where the pressure is lower. s the wing moves forward, it creates a spiral of air trailing off the wingtips. This spiral moves outward, upward, and inward, trailing behind the wing. Lower pressure (above wing) Drooped wingtips help create a Fig. 39 more efficient wing. Fig. 39 Higher pressure (underneath wing) Spiraling airflow Fig. 40 Now you have a basic idea of how parasite and induced drag affect an airplane and you re ready to take a closer look at how induced drag can fool you into believing your airplane is ready to fly when it isn t. Ground Effect The object of golf is to hit the ball the least number of times. Do this and you win. Recently, I was asked to play at a professional tournament. I used a new strategy. I stayed home, never hit the ball, and claimed victory. If wings can bend air, why can t I bend the rules of golf? Here s one of those nifty pilot nuggets (or nougats) with which you can win a hamburger some day. Is it possible to unbend the air which is bent by the wing at a large angle of attack? Yes, using the wizardry of something called ground effect. Ground effect allows an airplane flying close to the runway to remain airborne at a slightly lower-than-normal speed. During landing, this means the airplane might have a tendency to float (continue to glide just above the runway). During takeoff, the airplane can become airborne before it has sufficient speed to climb. Either event is like buying new glasses, taking them back because you can t read, then finding out you re illiterate. oth cause a lot of chagrin, but a takeoff like this can be dangerous. To understand how ground effect affects an airplane, you need to know something about how wind tries to escape around a wing. Remember that a wing generates lift because of increased pressure on its lower surface and decreased pressure over its upper surface. t high angles of attack, this pressure differential is quite large. Large pressure differences cause air molecules to act like lots of little prison inmates, with each molecule waiting for the slightest chance to break out of jail. Given that chance, air molecules underneath the wing would The perils of ground effect become obvious when you re too fast on the approach! gladly sneak over the top, where lower pressure resides. This reduces the wing s lifting efficiency. Unfortunately, it s difficult to design wings that preclude this escape. Figures 39 and 39 show several attachments added onto wingtips to thwart escaping air. Drooped wingtips are a common sight on many airplanes. They create a barrier to help contain escaping high pressure air from under the wing. Winglets are another common sight ttempting to climb out of ground effect at too slow a speed can cause the airplane to sink back onto the runway (if there is any runway left to sink back to at this point!). on larger airplanes. They also create a barrier that minimizes the escape of under-the-wing, high pressure air. Since they bend upward, they also give the impression that the airplane attempted to taxi between two closely parked gas trucks. ir sneaking from under the wing spirals upward and over the wingtip (Figure 40). Couple this spiral with air moving backward over the wing and you get a vortex similar to a horizontal tornado. Called a wingtip vortex, this tornado spirals outward, then upward, then inward behind each wing. You can expect this vortex action to increase at large angles of attack, where the pressure differential between the top and bottom of the wing is greater.

24 24 Rod Machado s Private Pilot Handbook If these vortices only affected the air at the wingtips, aerodynamicists would have many fewer design problems. Unfortunately, not only does this vortex spiral around the wingtips, it also adds a downwash or downward flow to the air behind and along the wing s span as shown by irplane in Figure 41. The wing itself also thrusts air downward. t higher angles of attack this results in the downward bending of the relative wind in the vicinity of the wing. This newly bent relative wind is often called the local relative wind. Recalling that lift is always perpendicular to the relative wind, the total lift now tilts rearward slightly. It does so to remain perpendicular to the newly bent relative wind. Part of the total lift is now acting in a rearward direction. This results in an increase in induced drag at high angles of attack. When the wing moves close to the ground (usually within one wingspan s length or less), its vortices, thus its downwash, diminish. In other words, the downwash can t shove the air into the ground. Voila, the local relative wind in the vicinity of the wing unbends as shown by irplane. This has the effect of allowing the wing s total lifting force to tilt upward slightly, despite the wing s large angle of attack. n upward tilting of the total lifting force means the airplane experiences less induced drag (less rearward pull of lift). This becomes quite evident when you compare irplanes and in Figure 41. The net result is that it takes less power to produce the necessary lift for flight while in ground effect. You win the hamburger. Where to Use Caution ttempting to emerge from ground effect before the plane is really ready to fly is like a newly hatched butterfly taking off before its wings are dry. It s not going to work. Trying to climb out of ground effect at this slower speed can cause the airplane to sink back onto the runway (if there s any runway left to sink back to at this point!). WING PERFORMNCE OUT OF GROUND EFFECT t large angles of attack, wingtip vortices result in a downwash behind the wing. This acts to bend the relative wind and tilt the total lifting force aft (the total lift force tilts so as to remain perpendicular to the bent relative wind). component of the total lift force, called "effective lift," acts vertically, opposite weight. This is the component responsible for keeping the airplane in the air. Induced drag is part of the total lifting force which pulls the airplane aft. This makes the airplane slow down quicker and prevents faster acceleration. Fig. 41 Components of total lift Total lift is tilted rearward WING PERFORMNCE IN GROUND EFFECT t the same large angle of attack within ground effect, the previously bent relative wind (in the vicinity of the wing) is straightened. Downwash can't blow into the ground. Therefore the total lifting force is tilted upward slightly to remain perpendicular to the relative wind. This acts to reduce the rearward pull of induced drag on the airplane, allowing the airplane to become airborne earlier and remain airborne longer (float) with less drag. Therefore, it's possible to float on landing or liftoff at too slow a speed for a safe climb. Downwash bends the relative wind "downward" in the vicinity of the wing. Components of total lift Relative wind straightened Wing Performance Out of Ground Effect Downwash blows downward on tail which pushes the Nose held up nose up. Eff. Lift Eff. Lift Drag Drag Total Lift Total Lift Total lift has tilted upward & points less aft producing less drag. GROUND EFFECT ND PITCH CHNGES Fig. 42 Nose wants to drop Wingtip vortices create downwash which helps tilt the total lifting force aft. Wingtip vortices, thus downwash, have decreased, allowing total lift to tilt forward. Wing Performance in Ground Effect Downwash diminishes in ground effect and no longer blows down on tail. This causes the nose to pitch down.

25 Chapter 2 - erodynamics: The Wing is the Thing 25 You must reach the airplane s minimum climb speed before charging out of ground effect. ecause of the rather large reduction in induced drag, ground effect can also cause the airplane to float during landing. Higher approach speeds make this particularly noticeable. Certainly this isn t much of a problem if you re landing at the onneville Salt Flats (don t try it). If, however, you re landing on a 1,000 foot strip and approach at excessive speed, you might need a contractor s license (you ll need it to rebuild all the houses, fences and sheds at the runway s end). Here s a big secret. If you re approaching at a speed above the normal approach speed, slow down before entering ground effect. Since ground effect becomes noticeable at a wing span s length and exists until touchdown, slow the airplane to the manufacturer s recommended approach speed before reaching this height. Continue your descent at this speed. HOW FLPS CHNGE THE WING'S CURVTURE Wing Slightly Curved FLPS UP Chord Line Wing Curved More FLPS DOWN Chord Line When flaps are lowered, the wing's curvature increases (surface area can increase too) and the chord line moves to increase the wing's angle of attack. This allows the wing to produce more lift for a given airspeed. Flaps can increase the wing s surface area by extending rearward and downward. Fig. 43 Different Designs On airplanes having low mounted tails, downwash from the propeller and the wings provides a downward acting force on the tail. Power reduction reduces this downwash and is followed by a nose-down pitch. In this respect, the airplane has less longitudinal stability since its pitch is affected THE "T" TIL T-Tail Is Unaffected by Wing & Prop Downwash Low Mounted Tail Lies in Wing & Prop Downwash by power changes. T-tail airplanes are less affected by this since the tail is above this downwash. dditionally, the T-tail provides one of the best configurations for spin recovery. Disadvantages are usually heavier weight for T-tail construction as well as complicated elevator control and trim linkages. Pitch Changes In and Out of Ground Effect s if floating on landing and an inability to climb after takeoff aren t enough of a problem, expect to experience subtle pitch changes when entering or leaving ground effect. s the airplane becomes airborne and flies out of ground effect, the wing s downwash increases. This blows air downward on the tail as shown by irplane in Figure 42. It s possible to become airborne without sufficient speed, then attempt to climb and have the nose pitch up slightly. This is the last thing you want to happen when the speed is low. If you re not prepared for this, it will be an unhappy surprise (and pilots don t like surprises!). During landing, as the airplane enters ground effect and the downwash diminishes, the nose tends to pitch forward. If you are not prepared for this you might land suddenly or sooner than expected. The secret to managing ground effect is to anticipate it. Expect ground effect to increase when the airplane is within a wing span s height above the runway. Low wing airplanes experience more ground effect than their high wing cousins. e prepared for a slight nose-up pitch during takeoff and a slight nose-down pitch when landing. Flap Over Flaps Ever wondered why the wings of large commercial airplanes sprout aluminum prior to takeoff and landing? Fast airplanes require small, thin wings to achieve the eye-popping velocity needed to satisfy today s speedhungry air traveler. The problem with thin, small wings is that they stall at a very high speed. Most jet airliners would have to take off and land at close to 200 MPH to achieve a safe margin above stall if they couldn t enlarge their wing s surface area and change their camber to create a temporary low speed wing. Engineers, however, design wings to do just that by supplying them with flaps. Extending or retracting flaps changes the wing s lift and drag characteristics. Lowering flaps lowers the trailing edge of the wing as shown in Figure 43. The wing s lift is increased in two ways. First, the lowered trailing edge increases the angle the chord line makes with the relative wind. Greater lift results from this increased angle of attack. Second, the lowered trailing edge increases the curvature on part of the wing resulting in increased air velocity over the wing s upper surface (many flaps even increase the wing s surface area by extending downward and outward).

26 26 Rod Machado s Private Pilot Handbook ecause of the larger angle of attack and greater curvature, flaps provide you with more lift for a given airspeed. In case you haven t noticed, this is also why flight control surfaces (ailerons, rudders and elevators) work. They change the curvature of the surface to which they are connected. This changes the lift produced by that surface. Flap Varieties Flaps come in several varieties (Figure 44). They all serve two basic purposes: to increase lift and drag. Fowler flaps () extend downward and backward. This provides a nice addition to wing curvature as well as increasing the surface area of the wing. Fowler flaps are common on smaller single engine Cessnas like the C-150, C-152, C-172 and C-182. Other varieties of flaps, like the plain type (), offer an increased wing curvature without an increase in the wing s surface area. Plain flaps are found on airplanes like the Grumman merican Yankee trainer. Slotted flaps (C), increase curvature of the wing and add fast moving air to the trailing edge of the wing (fast moving air helps prevent burbling and airflow separation which results in a stall). This type of flap is typical on airplanes like the Piper Warrior and Piper rcher. Split flaps (D), found on airplanes like the Cessna 310, increase the impact energy under the wing, thereby adding to the wing s lift (this impact energy results from the barn door lift we previously discussed). These flaps also add a great deal of drag to the wing. Why Use Flaps? What s the reason for putting flaps on small airplanes? First and foremost, they create the lift necessary to maintain flight at slower airspeeds. When landing, your goal is to approach and touch down at a reasonably slow speed. You certainly don t want to touch down at cruise speed. Such a high speed landing might just turn your tires into three little puffs of smoke. Flaps allow you to approach and land at a slower speed while maintaining a safe margin above the stall speed. slower speed on touchdown means less runway is used to stop. This is an important consideration if the runway is short. lternatively, if the wind is gusty, you might consider approaching with little or no flap extension. t the slower speeds allowed by flaps, the airplane becomes more difficult to control (the controls are not as responsive). Let s see how effectively the flaps increase lift by referring to the airspeed indicator (Figure 45). Since many flaps are painted white (we ll assume they are for this discussion), the airspeed indicator s white arc represents the flap operating range. The beginning of the white arc () is known as the power-off, full-flap stalling speed (in nonaccelerated flight at the airplane s maximum allowable weight). It s the speed at which the airplane stalls with flaps fully extended, power off and the gear extended. In Figure 45, the airplane will fly when 53 knots of wind blows over the wings if they are below their critical angle of attack. The high speed end of the white arc is the maximum speed you may fly with flaps fully extended. Flying beyond this speed can damage the flaps. In this example, you wouldn t want the airspeed indicator to indicate more than 107 knots with flaps extended (some airplanes, however, allow you to fly at a higher speed with partial flaps extended). ringing broken or bent airplanes back from a flight isn t a good idea, even if they are rentals (you ll find out how bad an idea it is when you get the bill for unbending the metal). Notice that the white arc () begins at a speed seven knots slow- Fowler flaps move rearward and downward thereby increasing wing area and wing curvature. High velocity airflow from underneath the wing, flowing up and over the flap, helps delay the onset of a stall. TYPES OF FLPS FOWLER FLP Plain flaps lower the trailing edge of the wing, increasing its curvature. This acts to increase the wing's lift. PLIN FLP C Slotted flaps allow high velocity airflow from underneath the wing to flow up and over the flap. This helps prevent airflow separation and delays the onset of a stall. SLOTTED FLP Split flaps generate some lift and a lot of drag since they disrupt airflow on the D underside of the wing SPLIT FLP 120 Fig. 44 FLP SPEED RNGE Flaps extended - 53 kts. (beginning of white arc) KNOTS 100 No flaps - 60 kts. (beginning of green arc) Green rc White rc Fig. 45

27 Chapter 2 - erodynamics: The Wing is the Thing 27 er than the green arc (). From an earlier discussion, we learned that the green arc is the power-off stalling speed with flaps retracted (gear retracted too). This airplane must have 60 knots or more of wind flowing over the wings to fly with flaps retracted. With flaps fully extended, you can touch down at a slower speed seven knots slower, to be exact (the full flap stall speed on the airspeed indicator assumes the airplane is at its maximum allowable weight). ut, as Confucius might say, Man who sow wild oats... eventually have crop failure. In other words, you don t get something for nothing. Flaps provide you with lift but they also produce drag. Full flaps create a very low speed wing. Try to accelerate it and at some point, drag defeats your efforts. Fortunately, the first half of flap travel usually provides more lift than drag. The last half usually provides more drag than lift. This is why some aircraft manuals recommend only 10 to 25 degrees of flaps for takeoffs on short fields (usually one or two notches on a three to four notch, manual flap system). If you re high while on approach to land, you can select full flaps to increase the airplane s drag. It s considered normal to use flaps only when descending within the traffic pattern and not when descending from cruise flight. fter all, cruise flight descents are efficient and fast at higher speeds where the parasite FLPS ND FORWRD VISIILITY OVER THE NOSE pplication of flaps in the traffic pattern allows you to have better visibility over the airplane s nose, making it easier to observe traffic and see the runway. drag is greater. If you wanted to descend with flaps from cruise flight, you d have to slow the airplane down below maximum flap extension speed (the top of the white arc) before applying flaps. This would be cumbersome. The airplane can descend faster at cruise speed with reduced power while getting you to your destination sooner. Since flaps provide more lift at slower speeds, think carefully about how and when they are retracted while airborne. If you re making a full flap approach and it s necessary to go around (i.e., give up this approach, climb, and return for another landing attempt), don t retract the flaps all at once! This would be like having someone remove a part of your wing at a slow speed. The sudden and often dramatic increase in stall speed could place There are several officially recognized forms of drag: THIS ISN T ONE OF THEM! You know you forgot to unhook the tail s tiedown chain and are towing the tiedown block when it takes full power just to taxi. Flaps Nice to Know Ideas Flaps you near a stall before you can accelerate to a safer speed. pply full power first, then retract the flaps in increments. In airplanes with 30 to 40 degrees of flap extension, retract the flaps to their leastdrag/maximum-lift position. Usually, this position is found at one-half flap travel (depending on the airplane). In airplanes with three notches of manually applied flaps, retract one notch first, followed by the other two once the airplane begins to accelerate. Do be careful in low wing airplanes when the flaps are up and someone is standing on the inboard section of the wing (be especially careful if you re airborne at the time you re in a Twilight Zone movie). friend was on the checkride for his private pilot s license when he damaged the F checkpilot. efore takeoff, the check pilot boarded the airplane, realized he forgot something and decided to return to the flight school. He opened the door, stepped out on the wing over an approved step as labeled on the inboard flap section. My friend, realizing that he hadn t checked the flaps during the preflight, decided to lower the handlever-activated flaps. n ahhhhhhh, then thud, were heard. The checkpilot went aerobatic and fell straight down. My friend, trying to break the tension with a little humor, looked over and said, Hey, while you re down there, how about checking those tires for me? On his second checkride

28 VERTICL COMPONENT OF LIFT TOTL LIFT WEIGHT WEIGHT 28 Rod Machado s Private Pilot Handbook How irplanes Turn There are many misconceptions in aviation. For instance, there are pilots who think propwash is a highly specialized detergent. nd a select few think that carburetor ice comes in three flavors. Sometimes pilots even have misconceptions about how airplanes turn. Let s examine what causes an airplane to turn, then look at how to perform this nifty little maneuver. s a young student pilot, an F inspector asked me how an airplane turns. I looked at him and said, With the wheel, sir. He clutched his chest and shook his head in disbelief. I admit that my answer was a little off and that he was a tad upset (the foam from around his mouth and his eyebrows merging with his hairline were good clues). Nevertheless, shouldn t I get partial credit? I guess Mother Nature doesn t grade on a curve. irplane in Figure 46 shows a view of an airplane in straight, wings-level flight. From this vantage point, lift acts vertically. Lift pulls upward on the airplane, keeping it suspended in flight. Surely, if lift can pull upward, then it can pull a little to the left or right. When it does this, the airplane turns. (To be precise about it, lift actually pushes upward on the wing but I ll use the term pull instead. It makes the concept of forces a little easier to visualize.) OK, so how do you make an airplane turn? If you said, With the wheel, I am deeply indebted and promise not to have a heart attack. In fact, turning the wheel or deflecting the control stick (i.e., banking the airplane with ailerons), is exactly how we tilt the total lifting force and start a turn (I ll explain control usage shortly). irplane shows the total lift force in a banked airplane. Part of the lift force pulls the airplane up (the vertical component of lift) and part pulls the airplane in the direction of the turn (the horizontal component of lift). You can use your imagination and visualize two separate and smaller forces making up the total lifting force. (There are those arrows again. You will not see these on a real airplane, so enjoy them while you can.) The arrows simply represent the forces of lift. If you re asked why an airplane turns, respond by saying that it s the horizontal component of lift that s responsible. The horizontal component of lift pulls the airplane in an arc. The larger the angle of bank, the greater the horizontal component and the quicker the airplane can turn. The horizontal component of lift is responsible for turning the airplane. Remember, you never get something for nothing. Tilting the total lift force while in a turn means less lift is available to act vertically against the airplane s weight, as shown by irplane in Figure 46. The airplane responds by moving irplane in Straight & Level Flight Fig. 46 HOW N IRPLNE TURNS irplane in a Turn TOTL LIFT HORIZONTL COMPONENT OF LIFT anking the airplane causes the lift force to tilt, which pulls the airplane in the direction of bank. Technically, it's the horizontal component of the tilted lift force that makes the airplane turn. RESULTNT FORCE Centrifugal Force in the direction of the momentarily larger force downward, in the direction of weight. We compensate for this by increasing our lift slightly whenever we enter a turn. y now, diligent student that you are, you know that applying back pressure on the elevator will increase the angle of attack which increases the airplane s total lift. Unfortunately, the increase in angle of attack also increases the drag which slows the airplane down. In a shallow banked turn (somewhere around 30 degrees or less) this decrease in speed isn t a concern. Steeper turns (45 degrees or more) may require the addition of power to prevent the airspeed from decreasing too much. Your job as a pilot is to keep The Four Forces acting on an airplane under control and in balance. To do so you need to know something about the airplane s flight control system. Longitudinal xis (Roll) Vertical xis (Yaw) THE THREE XES OF N IRPLNE Fig. 47 Hey! What switch did you touch? Lateral xis (Pitch)

29 Chapter 2 - erodynamics: The Wing is the Thing 29 HOW ILERONS NK THE IRPLNE ilerons are the moveable surfaces on the trailing edge of the wing. Fig. 49 Fig. 48 The aileron is the moveable appendage on the outer edge of each wing. Flight Controls If you re ready-made pilot material you ve been patiently licking your chops waiting for the discussion on flight controls. Gandhi would applaud your patience (but Gandhi isn t here, so I will). Figure 47 shows the three imaginary axes of the airplane. y use of the flight controls, the airplane can be made to rotate about one or more of these axes. The longitudinal or long axis runs through the centerline of the airplane from nose to tail. irplanes roll or bank about their longitudinal axis. sideways pass in football is called a lateral pass. Similarly, the lateral axis runs sideways through the airplane from wingtip to wingtip. irplanes pitch about their lateral axis. The vertical axis of the airplane runs up and down from the cockpit to the belly. irplanes yaw about their vertical axis. Think of yawing motion as yawning motion. In the morning you yawn by standing and stretching vertically, rotating right and left, waiting for those vertebra to kick in. Let s examine each of the three main flight controls that cause an airplane to move about its axes. ilerons Wheel turned right L L R R More lift with a lowered aileron Less lift with a raised aileron Less lift with a raised aileron More lift with a lowered aileron ilerons are the moveable surfaces on the outer trailing edges of the wings (Figure 48). Their purpose is to bank the airplane in the ngle of ttack direction you want to turn (Figure 49). When ngle of ttack the control wheel is turned to the right or the left, the ailerons simultaneously move in opposite directions. One raises and one lowers as shown by irplanes & C in Figure 49 (this doesn t mean they re broken either). ilerons work this way on purpose, allowing one wing to develop more lift and the other to develop less. Differential lift banks the airplane, which tilts the total lifting force in the direction you want to turn. lowered aileron increases the curvature, angle of attack and lift on a portion of the wing as shown by Figure 49D. Conversely, an upward moving aileron reduces the angle of attack, decreases the curvature and decreases the lift on a portion of the wing as shown by Figure 49E. (ny reference to control wheel also applies to airplanes with a control stick.) L L R Wheel turned left D R C lowered aileron increases the angle of attack and increases the wing's lift. Chord Line anking to the Right anking to the Left E raised aileron decreases the angle of attack and decreases the wing's lift. Chord Line

30 30 Rod Machado s Private Pilot Handbook While ailerons change the wing s lift, they also change its drag, and the change in drag is different for each wing. This results in the airplane s nose yawing in a direction opposite the direction of turn. Right turn, left yaw. This is referred to as adverse yaw, since it tries to turn the airplane opposite the direction the pilot intended to turn. dverse Yaw few old sayings are still floating around that I d like to revise. For instance, The pen is mightier than the sword. This is true unless you re one-on-one with a modern incarnation of Zorro. wonderful saying I wouldn t care to upend is, You can t get something for nothing. It sure works that way in aviation. When an airfoil develops lift, it s always paid for by an increase in drag. The lift produced by the aileron is no different. Downward moving ailerons create more lift and thus more drag (Figure 50). This results in adverse yaw. For instance, in a right turn, the downward moving aileron on the left wing creates more drag than the upward moving aileron on the right wing as DVERSE YW irplane banks to the right but nose yaws to the left. pplying right rudder yaws the nose of the airplane to the right. Tail moves to the left L R Right rudder applied L R HOW RUDDER COMPENSTES FOR DVERSE YW Low pressure Nose yaws to the right WIND Tail movement High pressure shown in Figure 53, and the airplane yaws (turns) to the left. similar and opposite effect occurs with a left control wheel deflection (Figure 53). dverse yaw presents a problem. You can t have the nose yawing or pointing in a different direction than you bank. Fortunately, airplanes have a control surface designed to correct for adverse yaw. It s called a rudder and is shown in Figure 51. Rudders The rudder s purpose is to keep the airplane s nose pointed in the direction of turn not to turn the airplane! Remember, airplanes turn by Vertical axis Fig. 53 pplying left rudder yaws the nose of the airplane to the left. High pressure Nose yaws to the left WIND Low pressure Tail movement Tail moves to the right L L R R Left rudder applied banking. Rudder simply corrects for adverse yaw and keeps the nose pointing in the direction of turn. Think of a rudder as a vertical aileron located on the tail of the airplane. right or left deflection of the rudder foot pedals located on the cockpit floor (Figure 52) changes the vertical stabilizer s angle of attack and yaws the airplane about its vertical axis. This yawing motion keeps the airplane s nose pointed in the direction of turn. pplying right rudder pedal, as shown by irplane in Figure 53, forces the tail assembly to swing in the direction of lower pressure. s the tail moves, the airplane rotates Lowered aileron creates more lift but it also increases drag. The Rudder irplane banks to the left but nose yaws to the right. Fig. 50 lowered aileron creates more lift but it also increases drag. Fig. 51 Rudder Pedals Fig. 52

31 Chapter 2 - erodynamics: The Wing is the Thing 31 THE TURN COORDINTOR Turn needle L Fig. 54 DC ELEC NO PITCH INFORMTION TURN COORDINTOR 2 MIN about its vertical axis. pplication of right rudder pedal yaws the nose to the right. pplying left rudder pedal (I ll just say rudder from now on), shown by irplane, yaws the nose to the left (surprising, huh?). irplane Slipping: Nose is pointed outside the turn. Inclinometer The movement of the ball corresponds to the movement of the sunglasses on your car's dashboard. The same force that moves the glasses also moves the ball. The ball, however, slides more easily than the glasses. The ball's deflection from center identifies when the airplane's nose is pointed other than in the direction of turn. Rudder is used to move the ball back to the centered position. R When should you use the rudder? ny time you turn the airplane. If you don t use rudder while trying to turn, part of the airplane is going one way, and another part points in the opposite direction. This is not a pretty sight, and your instructor s eyebrows will raise so high that they will scratch his or her back. Right turn, right rudder. Left turn, left rudder. Feet and hands move together. Now the question foremost in your upper brain is How much rudder is enough? Good question. Figure 54 shows an inclinometer, also known as the ball, as a part of another instrument called the turn coordinator (located on the instrument panel). The little white airplane in the turn coordinator shows the direction of turn, while the ball tells you if the proper amount of rudder is applied. The ball is free to roll right or left within the glass tube. ny inappropriate rudder use (or lack of use) applies an unnecessary side force to the airplane. This deflects the ball in much the same way sunglasses scoot across your car s dash when rounding a sharp corner. Your job is to keep the ball centered by using the rudder. Figure 55 shows an airplane in a turn. irplane s nose is pointed outside the turn (probably because of insufficient right rudder or too much N ESY WY TO UNDERSTND SLIPPING & SKIDDING IN N IRPLNE irplane Flying Coordinated: Nose is pointed in direction of turn. SLIPS & SKIDS MDE ESY n easy way to understand slips or skids is to think of the ball as the tail and the glass tube at the wings of the airplane. (Right turn shown.) Wings irplane Skidding: Nose is pointed inside the turn. Tail In a right turn, if the ball (tail) is to the left of center, this implies the tail is skidding to the outside of the turn. In a right turn, if the ball (tail) is to the right of center, this implies the tail is slipping to the inside of the turn. C Glasses move to right Glasses stay centered all in the inclinometer is forced to the right, just like sunglasses. DC ELEC all in the inclinometer stays centered just like sunglasses. DC ELEC all in the inclinometer is forced to the left, just like sunglasses. Glasses move to left DC ELEC TURN COORDINTOR L R 2 MIN Right rudder necessary TURN COORDINTOR L R 2 MIN Correct rudder application TURN COORDINTOR L R 2 MIN Fig. 55 Left rudder necessary

32 32 Rod Machado s Private Pilot Handbook right aileron is applied). The ball and the airplane slip to the right, toward the inside of the turn. In other words, you need to point the nose slightly to the right for a precisely aligned turn. y adding enough right rudder to align the airplane in the direction it s turning, the ball returns to the center as shown by irplane. irplane C s nose points toward the inside of the turn (probably because too much right rudder is applied or insufficient right aileron is used.) The ball and the airplane skid to the left, toward the outside of the turn. dding a little left rudder keeps the nose pointed in the direction the airplane s turning and centers the ball. Simply stated, if the ball is deflected to the right or left of center, add enough right or left rudder to center the ball. Sometimes you ll hear your instructor say, Step on the ball! This is simply your instructor s way of telling you to add right rudder for a right-deflected ball and left rudder for a left-deflected ball. Don t even think about placing your foot on the turn coordinator, or your instructor will question you about your ST scores. Don t put marbles in your shoes either. When entering a turn, aileron and rudder are applied simultaneously, in the same direction. This is what pilots mean when they refer to flying coordinated. ileron establishes the degree of bank and rudder keeps the nose pointed in the direction of turn. If the ball is centered during this process, we say that the controls are properly coordinated. One last point about the rudder. It is the last control forfeited in a stall. Even when the airplane is stalled, it continues to move forward with airflow over its surface. This allows some degree of rudder authority even at very slow speeds. During a stall you ll find that the rudder is very effective for maintaining directional control. In a spin, the rudder is very important for spin recovery. More on spins in Postflight riefing #2-1. Devious Instructor Device Hey! What about two boxing gloves suspended by string that act like the ball in the inclinometer? Pulling back on the control wheel deflects the elevator upward which forces the tail downward. This, in turn, causes the nose to pitch up. Fig. 56 Pushing forward on the control wheel deflects the elevator downward which forces the tail upward. This, in turn, causes the nose to pitch down. Coordination Lube HOW THE ELEVTOR CONTROL CHNGES THE IRPLNE'S PITCH Tail moves down & nose moves up Tail moves up & nose moves down It won t take long before you won t need to look at the ball in the inclinometer to know whether or not you re flying coordinated. With a little practice you ll be able to fly by the seat of your pants (assuming you wear pants while flying). Pressure on the right or left side of your derriere is caused by the same force that moves the ball to the right or the left. It doesn t matter how big the airplane is; a 747 or a Cessna 150 Land-o-matic can be flown by the seat of the pants. Use the inclinometer as sort of a biofeedback device cuing you to sense pressure on your derriere when the ball is deflected. nd don t feel bad if it takes a little practice to get used to coordinating rudder and ailerons. My first instructor told me that even if they put cold Vaseline in the inclinometer (instead of mineral oil), I would still get the ball to bang back and forth against the ends of the glass. Tail movement (down) Tail movement (up) Elevator The elevator is the moveable horizontal surface at the rear of the airplane. Its purpose is to pitch the airplane s nose up or down. The elevator control works on the same aerodynamic principle as the rudder and aileron. pplying back pressure on the control wheel of irplane in Figure 56 deflects the elevator surface upward. Lower pressure is created on the underside of the tail which moves it downward, and the nose of the airplane pitches up.

33 Chapter 2 - erodynamics: The Wing is the Thing 33 nother version of the trim tab is a single tab spanning the stabilator. The Elevator Trim Wheel Some elevator trim tabs are a single tab on one side of the elevator. Fig. 57 Fig. 57 Fig. 57C irplane shows what happens when the control wheel is moved forward. The elevator surface moves down creating lower pressure on the top side of the tail. This causes the tail to rise. The nose rotates about the lateral axis in a downward direction. Simply stated, to pitch up, pull the control wheel back; to pitch down, move the control wheel forward. Trim Tabs If you had to apply continuous pressure on the control wheel to maintain pitch attitude, your arms would tire quickly (Schwarzenegger would be proud of you but I wouldn t). Fortunately, airplanes have something known as a trim tab to take the pressure off the control wheel (and off the pilot!). trim tab is a small, moveable surface attached to the main surface you want to control (in this case, it s the elevator. ilerons and rudder can have trim tabs too). Figures 57, 57 and 57C show two different types of trim tabs and the trim wheel used to change the trim tab s position (the wheel is usually located between the two front seats or the lower portion of the instrument panel). Moving the trim tab creates a slight pressure difference on the very end of the control surface to which it s attached (Figure 58). Just enough pressure is created to keep the primary control surface in the desired position without having to hold the control wheel in place. Notice that the trim tab moves in a direction opposite to the primary control surface it affects. If you want the elevator to deflect upward (as if you re pulling back on the wheel in a climb), the trim tab must move down as shown by Elevator. To maintain a downward deflection of the elevator (as if you re in a descent), the trim tab must move upward as shown by Elevator. Using elevator trim is quite simple. Select the attitude desired with the elevator, then rotate the trim wheel (up or down) to take the pressure off the control wheel. How do you know which way to twist the trim wheel? Most trim wheels say nose down or nose up above or below the wheel. Simply twist the wheel in the direction you want the nose to stay. ileron and rudder trim are equally simple to use. When flying with other pilots, accept the fact that they won t like the way you trimmed the airplane. You might have flown the last 200 miles without touching the plane. Hand it over to the other person and the first thing he or she does is start fiddling with the trim. I can only conclude that this action is a form of primitive territorial claim. It s annoying, but it s better than how wild animals claim their territory isn t it? pplying nose-up trim moves the tab down, creating a small low pressure area on the end of the elevator. This causes the elevator to move upward. Upward lift caused by trim tab Fig. 58 HOW ELEVTOR TRIM WORKS The trim wheel is usually located below the throttle in the center of the airplane. Nose down Nose up Lift acting downward caused by trim tab pplying nose-down trim moves the tab up, creating a small low pressure area on the bottom tip of the elevator. This moves the elevator downward.

34 34 Rod Machado s Private Pilot Handbook Left Turning Tendencies (not political) Having mastered The Four Forces, you now need to know there are others. Not many others, and not as significant, yet they are forces to be reckoned with. Torque Newton (Isaac, not Wayne) once said that for every action there is an equal and opposite reaction. Crankshafts rotate to the right in most single engine airplanes (as seen from the cockpit). n equal and opposite reaction is for the airplane to rotate to the left. That s exactly what the airplane does when the engine is developing power. This is known as engine torque or just torque, as shown in Figure 59 (I ve often wondered if Spanish instructors call this Torquemada?) Expect the effect of torque to be greatest when power is at a maximum and the airspeed is slow. During the takeoff run, with maximum power, torque makes the airplane want to roll to the left, creating slightly greater pressure on the left tire. This creates a little more friction on the left tire and causes the airplane to turn to the left as it accelerates for takeoff (as you ll soon see, there are other forces that aid in this left-turning tendency). Once airborne, the airplane still wants to roll to the left because of torque. Pilots can compensate for torque by the applying the appropriate rudder and aileron control inputs. This is one reason why you need a lot of right rudder on takeoff (there are other reasons as you ll see shortly). Use of rudder and aileron trim also helps when the airplane wants to go somewhere other than straight during a climb. Torque is only one reason most airplanes like to turn left as they accelerate for takeoff. There are other forces that yaw an airplane to the left during slow speed, high angle of attack Propeller rotation SLIPSTREM EFFECT Yaws left The airplane yaws to the left as a result of the propeller slipstream. Slipstream Push on tail Propeller rotation efore we proceed further, you should understand that the propeller is nothing more than a wing moving around a crankshaft. Fig. 60 ENGINE TORQUE RECTION irplane reaction irplane rolls to left as a result of engine power. Fig. 59 flight. The effect of the propeller slipstream is one of these. Slipstream Effect clockwise rotating propeller (as seen from the cockpit) imparts a curve or spiral motion to air flowing past the fuselage. This is known as the propeller slipstream. Figure 60 shows how this swirling air affects the airplane. Under high power conditions (such as during takeoff or climb), a curved spiral of air swirls around the airplane striking the vertical stabilizer and the rudder, yawing the airplane s nose to the left. irplane design, power usage and airspeed all determine how the slipstream affects the airplane. We now have two forces that separately roll and yaw our airplane to the left. This explains why flight instructors are always saying three things: More right rudder, Let go, I ve got it, and Do you want to pay me now? Your instructor s admonition to use more right rudder to counteract the left-turning tendency is quite common during slow speeds and high power settings. las, there is still one more force that yaws an airplane to the left. P-Factor No, this is not something you experience on a long cross country flight. Often called asymmetric disk loading, P-factor (or propeller factor ) is one more thing that makes your airplane turn left. Think about the word asymmetric. It means not symmetrical, or not spread out evenly. symmetric disk loading occurs when lift, produced by the propeller (which is a rotating disk), isn t evenly distributed. efore we proceed further, you should understand that the propeller is nothing more than a wing moving around a crankshaft. While airplane wings develop lift when pulled through the air, propellers develop their lift when spun around a crankshaft. Each half of the propeller is

35 Chapter 2 - erodynamics: The Wing is the Thing 35 like a wing, and it follows all the laws that a wing must follow (i.e., the greater the angle of attack, the greater the lift and drag for a given speed, etc.). If one side of the propeller develops more lift than the other, the airplane experiences a twisting or yawing motion. This causes one side to pull more than the other, inducing a yaw about the vertical axis. P-factor is noticeable at high angles of attack for one very important reason: the downward moving half of the propeller develops more lift than the upward moving half. Figure 61 shows an example of this. t a low angle of attack (), both the upward and the downward moving propeller blade strike the air at similar angles. t higher angles of attack (), the downward moving blade has a larger angle of attack than the upward moving blade. This means the downward moving blade creates more lift than its upward moving companion (Figure 62). Therefore, the downward moving blade imparts a yaw to the left about the vertical axis. Figure 63 is your last and final way of visualizing P-factor (honest!). irplane is at a high angle of attack and is moving toward you. Its downward Fig. 62 SYMMETRICL DISK LODING (P-FCTOR) Prop lift Prop Rotation Prop lift t a low angle of attack, lift is evenly distributed on both the upward and downward rotating side of the propeller. Prop Rotation Relative Wind Relative Wind NOTHER LOOK T P-FCTOR When the airplane is at a low angle of attack, the rising and falling blades of the propeller have almost the same angle of attack. In other words, each half of the propeller produces an equal amount of lift and there is little or no P-factor. irplane at a low angle of attack Relative Wind Fig. 61 ngle of attack of the falling blade ngle of attack of the rising blade ngle of attack of the falling blade ngle of attack of the rising blade Propeller Shaft When the airplane is at a higher angle of attack, the falling blade has a larger angle of attack than the rising blade. This difference in angle of attack between blades results in the falling blade producing more lift than the rising blade. Thus, P-factor increases and the airplane yaws to the left. irplane at a high angle of attack Relative Wind Propeller Shaft NOTE! ll angles are exaggerated so you don't have to use your imagination! More lift Left yaw Less lift t higher angles of attack, lift increases on the downward rotating side of the propeller and decreases on the upward rotating side. This causes the airplane to yaw to the left. Hey, this is great for takeoff, but what about landing?

36 36 Rod Machado s Private Pilot Handbook moving blade swings forward into the wind. The upward retreating blade rotates with the wind. The downward moving blade feels more wind blowing on it than the upward moving blade. It is reasonable to conclude that the blade moving downward develops more lift than its rising counterpart. This is exactly what happens. irplane shows the net result of this lift differential as a left yawing tendency. Higher angles of attack increase this difference in lift, while lower angles of attack decrease it. Expect to notice P-factor at higher angles of attack, such as during takeoff and climb. Couple this with the slipstream effect and torque, and it certainly seems like a left-wing conspiracy. t slow speeds and high power settings, a healthy amount of right rudder is necessary to keep the airplane from yawing to the left in this condition. I fully expect that genetic engineers will, one day, create an entirely new breed of pilot known as Homo torquedown man. This hybrid species will have an enormous muscular right leg useful in correcting for torque, slipstream and P-factor when climbing airplanes without rudder trim. Hopefully, this chapter has helped you understand how to make the wing of your airplane work better for you. Don t worry if you don t feel like you quite understand these concepts yet. Seeing is believing, and it almost always takes a few flight lessons before the practical realities of aerodynamics sink in. Learning aerodynamics is like eating vegetables they re good for you, even if you re not sure why. It s like my little nephew s first introduction to broccoli. My sister used guilt as a motivator to get him to eat the stuff. She d always say, rad, eat your broccoli. Why? ecause there are children in China who are starving. Little did he know that the parents of children in China were saying, Chan, eat your rice. Why? ecause there are children in merica who have to eat broccoli. Hopefully this chapter was a little more pleasant than eating broccoli. The more you fly, the more you ll appreciate how important these concepts are. t a positive angle of attack, the downward moving blade develops more lift than the upward moving blade. This causes the airplane to yaw to its left. DOWNWRD MOVING LDE Downward moving propeller encounters wind blowing on it, thus providing more lift on this side. Prop movement Prop movement More lift Wind Now that you know how an airplane flies, we ll turn our attention to the item that pulls an aircraft through the air in the first place the engine. Wind SYMMETRICL DISK LODING (P-FCTOR) lade viewed from the tip irplane moving toward you at a large angle of attack UPWRD MOVING LDE Upward moving propeller encounters wind blowing with it, thus providing less lift on this side.* Prop movement P-Factor causes the airplane to yaw to the left when it is flown at higher angles of Left yaw Wind Prop movement *Note: The angle of attack of the upward moving blade is actually less than the downward moving blade (this is difficult to show, therefore, I simplified the concept). This difference in angle of attack is responsible for the differential lift produced by the propeller at high angles of attack. Less lift

37 Chapter 2 - erodynamics: The Wing is the Thing Stall, yaw & wing drop Postflight riefing #2-1 HOW SPIN OCCURS Hey, wanna see something neat? THE FULLY DEVELOPED SPIN Spin xis Incipient stage of spin Fully developed spin Spin recovery During the incipient stage of the spin the airplane stalls then commences to autorotate. t this point the nose may pitch straight down and the airplane may be slightly inverted. s the autorotation continues, the aerodynamic and inertial forces attempt to balance and settle the airplane into a predictable pattern of rotation. The incipient stage usually consists of the first two turns and takes five to six seconds to complete in most light airplanes. In the fully developed spin, the rotation, airspeed and vertical speed stabilize. The descent path is nearly vertical as the airplane pivots about the spin axis. Despite the steep nose-down attitude, both wings are stalled. The higher wing is less stalled than the lower wing. Consequently, the higher wing produces slightly more lift and the lower wing produces slightly more drag. This helps establish the autorotating motion of the spin. Unless something is done to unstall both wings, the airplane will remain in the spin. Recovery occurs when autorotation is stopped by reducing power (if not already reduced), neutralizing ailerons and applying opposite rudder. The elevator is moved forward to decrease the angle of attack (as it is during a normal stall recovery). Use caution. High airspeeds are possible during recovery. void overstressing the airplane. Outside wing: more lift & less drag (not as stalled) Inside wing: more drag & less lift (fully stalled) To enter a spin the airplane must first be stalled. s the stall occurs, rudder is applied and the Motion elevator is pulled back to further increase the angle of attack. pplying rudder (to the left in the above example) causes the left wing to simultaneously move downward and rearward. This causes a left roll that increases the left wing's angle of attack and slows its speed relative to the right wing. The right wing, on the other hand, moves upward and forward. This decreases its angle of attack and increases its speed. Despite the fact that both wings are stalled in the fully developed spin, the outside (higher) wing is less stalled than the inside (lower) wing. Pilots making skidding turns to final put themselves in a critical position that's conducive to spin entry. If the pilot sees he will overshoot the turn to final, he might apply rudder to align the nose with the runway while holding the bank constant with aileron. ll the while, he's pulling back on the elevator control in an attempt to keep the nose up. This is the precise condition described above for entering a spin. Unfortunately, when a spin occurs in the traffic pattern, there is often insufficient altitude available for recovery. In a fully developed spin, the airspeed remains constant (for smaller airplanes it's about 65 to 75 knots). The descent rate can exceed 7,000 feet per minute. Spin axis No!

38 38 Rod Machado s Private Pilot Handbook Postflight riefing #2-2 Your irplane-what a Drag Drag is a passenger that doesn t chip in for lunch. It s also a constant companion on every flight. Drag is not something a pilot has a great deal of control over, but understanding what the two types of drag are, and how they accumulate and affect flight, is important in certain situations. Parasite Drag parasite is usually something you don t like that seems to attach itself to you or something you value. large leech that bolts itself onto your forehead is a parasite. You don t like it, but it likes being connected to you (it is, however, good for sparking conversations at the airport). n airplane s struts, landing gear, wires and antennas (Figure 64) are all parasites as far as an aerodynamicist is concerned, and the drag they create is referred to as parasitic drag. These protruding parts grab air molecules and slow airplanes down like flies doing touch-and-goes on No- Pest strips. These and other parts of the airplane are, however, necessary aerodynamic evils. Fortunately, clever engineers work remarkable magic in reducing the effect these parasitic items have on airplane performance. Spoiler Spoilers can be extended and retracted from inside the cockpit. They project upward into the airstream destroying some of the wing s lift. Higher performance airplanes, because of their low drag profile, often find these devices useful in aiding their descents. LNDING GER ND NTENNS RE PRSITE DRG You might wonder, Why talk about drag if the pilot can t do anything about it? Well, first off, it gives you something to debate with other pilots on a day when you can t go flying. Second, not many people understand this stuff, so it makes you sound cool. Third, pilots do have some control over an airplane s parasite drag. For instance, polishing the surface of the wings minimizes skin friction (one of several forms of parasite drag). On certain high performance sailplanes, a good shine job can be good for several miles of increased glide performance. On training airplanes, the performance increase can be close to negligible. Nevertheless, it s a good excuse for a Saturday morning s escape to the airport. Curious spouses are simply told that important aerodynamic modifications are to be performed on the airframe (don t blink, look serious, control breathing and you ll pull this off). Nothing can (or should!) be done about the struts on an airplane having them. They re holding the wings on, which is generally considered a good thing. Remember, no wings, no lift. No lift, no fly. No fly, no fun. Unfortunately, struts clobber air molecules and increase parasite drag. Flying airplanes with struts and fixed gear has its advantages and disadvantages, and parasite drag is definitely one of the disadvantages. Remember, parasite drag rises dramatically with increasing speed. In fact, doubling the speed of the airplane quadruples the parasite drag. Figure 65 shows how parasite drag increases with speed. You might get the impression that at slower speeds the airplane experiences very little total drag. While some parasite drag is always present at slower speeds, a different form of drag actually increases with a reduction in speed. This is called induced drag. Low - PRSITE DRG - High Slow PRSITE DRG Fast Slow - IRSPEED - Fast Fig. 65 Fig. 64

39 Chapter 2 - erodynamics: The Wing is the Thing 39 INDUCED DRG ND NGLE OF TTCK NO LIFT Relative Wind When your hand has a "zero" degree angle of attack with the relative wind, no lift and very little drag is produced. The total lift tilts aft so as to remain perpendicular to the locally bent relative wind. Relative Wind Total Lift Things you're not likely to hear in aviation: 1. "Space shuttle on final, be prepared for a go around." 2. "Will the Piper Cub that's downwind please follow the jet and don't overtake it like you did the last time!" 3. "I know this is only your first lesson, but why don't you take her around the pattern a few times by yourself." 4. "No, I really don't have that much flight time." 5. "I understand everything the controller says." 6. "I like studying the Federal viation Regulations." 7. "I made megabucks renting my plane." 8. "Tower, this is 2132 ravo, I've got a phone number for you and I want you to give me a call when I land." Professor ob ig Kahoona s the hand's angle of attack increases, the relative wind in the local vicinity of the hand (underneath it) is bent slightly. The total lift the hand produces is now tilted slightly rearward so as to remain perpendicular (90 degrees) to this newly-bent relative wind. C The local relative wind is bent down by the hand. Relative Wind Large amount of induced drag present. Effective lift is acting 90 degrees to the undisturbed relative wind ahead of the hand. Total Lift s the hand's angle of attack increases, the local relative wind is bent even more. This acts to further tilt the total lifting force aft. The component or portion of the total lift acting vertically is called "effective lift." The portion acting rearward is called "induced drag." It's very important to notice that the larger the angle of attack, the more rearward the total lift acts, thus the greater the induced drag becomes. Yet, the effective lift still acts 90 degrees to the undisturbed relative wind ahead of the wing. Fig. 66 Induced drag Induced Drag People can sometimes be induced to act in an unfortunate way. This occurs when a TV commercial recommends that you purchase a do-ityourself acupuncture kit. fter a little experimentation, you discover Effective lift that you don t feel any better but your body is capable of picking up cable TV. Wings aren t people, but sometimes they induce the lift they produce to act in unfavorable ways. In particular, wings can induce their total lifting force to act rearward instead of in an upward direction. Remember, any force acting rearward on the airplane acts like drag. (t this point I ll offer you a choice. You can either believe me when I say that induced drag increases when the airplane slows down or you can plow through the following few paragraphs and I ll prove it to you. Most pilots can and do fly safely without knowing the nuances of drag formation.) Time for another experiment (yes, back to the car again). Stick your hand (and only your hand) out the window as you re moving. Hold your hand parallel to the relative wind so that it has little or no angle of attack as shown by Figure 66. You should feel little or no lift and relatively little rearward drag. Now, increase the hand s angle of attack as shown by Figure 66. Notice how your hand wants to rise (we call this lift) but it also wants to move rearward slightly (we call this drag). s the hand s angle of attack increases, the local relative wind in the vicinity of the hand is bent downward slightly. Earlier we learned that lift acts perpendicular to the relative wind. Therefore, the total lifting force is tilted rearward as the hand s angle of attack increases. further increase in the hand s angle of attack, as shown by Figure 66C, increases the upward and rearward pull of the total lifting force. Here s the point (finger in the ear again!). The total lifting force acting on the hand pulls it upward and backward as the angle of attack increases. The upward, or vertical portion of the total lift force works against the airplane s weight to keep it aloft. The rearward pull of the total lifting force acts in the direction of drag; we call it induced drag. lthough hands are much less sophisticated in generating lift than

40 40 Rod Machado s Private Pilot Handbook are wings, there are similarities between them. In Figure 67, Wing is at a low angle of attack. Its total lifting force is nearly perpendicular to the relative wind and acts nearly opposite the airplane s weight. s Wing s angle of attack increases, downwash from the wing bends the relative wind slightly in advance of the moving wing. The total lifting force tilts aft so as to remain perpendicular to the newly-bent, local relative wind as shown by Wing C. We can take the portion of the tilted lift force acting straight up, opposite the airplane s weight, and call that component the effective lift. It s the part that s effective in keeping us airborne since it acts opposite the airplane s weight. This effective lift also acts at a 90 degree angle to the Low - INDUCED DRG - High Fig. 67 Small angle of attack Relative Wind t small angles of attack the airfoil's total lifting force acts nearly 90 degrees to the relative wind. In other words, total lift acts upward without much of a backwards pull on the airplane. Similarly, this total lift force acts nearly opposite and parallel to the weight of the airplane. Slow 90 o INDUCED DRG Total Lift Weight Fast Slow - IRSPEED - Fast Fig. 68 TOTL LIFT, EFFECTIVE LIFT ND INDUCED DRG Increasing angle of attack Local relative wind bent downward by the wing's downwash. Relative Wind Local Relative Wind s the angle of attack increases, the wing actually bends the relative wind (in its local vicinity) downward slightly. The downwash created by the wing at this high angle of attack is responsible for bending the local wind underneath and slightly ahead of the wing downward. The bent relative wind is called the local relative wind. undisturbed relative wind ahead of the airplane. The horizontal portion of the tilted lift force acts rearward, in the direction of drag. We call this rearward pull induced drag. Obviously, when the angle of attack is small, the total Fuel Tank Professor ob carefully points out that max. endurance also applies to the human bladder since it s smaller than most fuel tanks. Professor ob ig Kahoona lifting force acts in more of an upward or vertical direction with little rearward pull on the airplane. s the angle of attack increases the drag induced by the rearward component of total lift increases. Consequently, when the airplane slows, the angle of attack increases and induced drag increases. s the airplane s speed increases, the angle of attack decreases and the induced drag decreases. This relationship between induced drag and airspeed is shown in Figure 68. Sort of wish you d just taken my word for it, don t you? Large angle of attack "Effective lift" acts perpendicular to the relative wind. Relative Wind C 90 o Induced drag Total Lift t larger angles of attack, the total lifting force tilts backwards with the wing. The component of lift acting parallel and opposite to the weight is effective in keeping the airplane airborne.thus, this vertical component of lift is called effective lift. The component of lift acting rearward is called induced drag. 90 o Maximum Endurance nd Range Maximum Range If you are ever over water with limited fuel, trying to make the shoreline, you ll gain a fuller understanding of just how important it is to know the speed for attaining maximum range. Most new pilots believe that they can simply add more power and try to get to the shoreline quicker. You will soon discover that in aviation, the first thing that makes sense is often the last thing that s likely to work. Faster speeds mean greater parasite drag. You end up going faster, at the tremendous cost of an increase in fuel consumption because drag has increased. The end result is that you land short of the maximum distance you would have traveled at the best L/D speed. Some pilots think they can slow the airplane way down close to the stall speed to conserve fuel. This doesn t work either. Remember, when the airplane slows below the best L/D speed, induced drag increases. You ll simply run out of fuel without having traveled the maximum distance possible at the best L/D speed. Effective lift Weight Total lift acts to the local relative wind Postflight riefing #2-3

41 Chapter 2 - erodynamics: The Wing is the Thing 41 If you re ever trying to make a destination on limited fuel, use the airplane s maximum range speed. Unfortunately, this is often not published in your airplane s Pilot s Operating Handbook (POH). If you can t find the maximum range speed, use the airplane s best L/D speed (Figure 69). (Once again, aerodynamics has its catches and qualifications. In theory, best range should be precisely the speed for minimum total drag the best L/D speed. However, based on propeller efficiency considerations, the maximum range speed can sometimes be a little higher than the posted best L/D speed for that airplane. Practically speaking, this doesn t amount to all that much of a difference.) Maximum Endurance Maximum endurance occurs at a speed where minimum power (thus minimum fuel consumption) is required to remain airborne. This is the speed that allows you to remain aloft for the longest possible time. The maximum endurance speed becomes an important consideration when you re circling above the only airport within range while waiting for the weather to clear. The last thing you ever want to be is low on fuel (Oops! Excuse me. To be politically correct we now say fuel challenged ). I hope you ll never find Low - TOTL DRG - High RNGE ND ENDURNCE SPEED FOR TYPICL HIGH PERFORMNCE IRPLNE Ind c u ed Drag Max. Endurance Max. Range Total Drag as 97 knots 74 knots Par i te Drag Slow - IRSPEED - Fast Fig. 69 Thrust Horsepower POWER REQUIRED VS. IRSPEED GRPH Engine Power - RPM RPM HUNDREDS RPM HUNDREDS Minimum power required at 74 kts. Power RPM HUNDREDS irspeed in Knots required to maintain altitude Point () is the lowest point on the power curve. Fig. 70 yourself in this situation. Not knowing anything about your airplane s maximum endurance (minimum fuel consumption) speed is like using a cast iron commode on the dark side of an iceberg a very uncomfortable experience. t first glance, it might appear that the speed for maximum endurance is the best L/D speed. fter all, minimum drag at the best L/D speed should require minimum thrust, thus minimum power. This might be true if it weren t for the fact that parasite drag and induced drag affect the power requirements for level flight differently. Simply stated, the engine must work harder in overcoming parasite drag (as you speed up) than in overcoming induced drag (as you slow down). Therefore, flying a little below the best L/D speed, even at the expense of increasing induced drag, lowers the power required to sustain level flight (up to a point of course). That s why you ll find the speed for maximum endurance (minimum power) below the minimum drag (best L/D) speed. The speed for maximum endurance (minimum power) is generally less than the best L/D speed (it s approximately 75% of the best L/D speed for most propeller driven airplanes, although this percentage can vary considerably from airplane to airplane). Think of this as you would the difference between gross and take home pay one of them (maximum endurance speed) is smaller than the other (best L/D speed). Figure 69 shows these two speeds. In some cases airplanes with long thin (high aspect ratio) wings have maximum endurance speeds that are very close to their stall speeds. This makes maximum endurance speed in these airplanes impractical to use. The maximum endurance speed can be seen more clearly on the power vs. airspeed graph (Figure 70). This chart compares the power required to maintain altitude at different airspeeds. t the lowest portion of the power curve (), the airplane maintains its altitude with minimum power, thus minimum fuel consumption. speed of 74 knots establishes the minimum power point (maximum endurance speed) for this airplane. irspeeds above or below this speed require that you increase power to maintain altitude. Figure 71 shows maximum endurance speed occurring below the minimum drag speed as shown by the drag curve inserts in the top of the figure. The power-required curve also provides a valuable clue as to how the airplane responds in a descent. Let s say you re descending at 74 knots

42 42 Rod Machado s Private Pilot Handbook Low - Thrust Horsepower - High Ind c u ed Drag Total Drag (the bottom of the power curve) at a slightly lower power setting than required to maintain altitude. Let s also say that this lower power setting produces a 500 foot per minute descent rate in this particular airplane. This is the minimum rate of descent you can expect in this airplane at this power setting. ny decrease in airspeed will increase the rate of descent. Why? The power required to maintain the descent rate increases as the airspeed decreases. This is a very important point! If you haven t heard about it yet, you will eventually hear about an airplane operating behind the power curve or operating in the region of reversed command. Either of these statements means the pilot is flying at a speed less than the minimum power-required speed. decrease in airspeed requires more power to hold altitude or maintain a desired rate of descent (this is reversed from what you normally expect and explains why it s called the region of reversed command ). Without this addition of power the airplane starts to descend or descend more quickly, as shown in Figure 72. There comes a point where an airplane flying behind the power POWER REQUIRED VS. TOTL DRG r g Parasite D a Ind c u ed Drag Total Drag Parasite D a Max. endurance (min. power) speed is less than the minimum total drag (best L/D) speed.* Increasing induced drag Power Min. power r g Total Drag Parasite D a Low - irspeed in Knots - High Ind c u ed Drag Increasing parasite drag r g *This occurs because the required engine power is more affected by increasing parasite drag than increasing induced drag. Therefore, the required to maintain altitude minimum power required speed is found to the left of the minimum drag speed, at a point where there is slightly less parasite drag. curve is unable to hold altitude, even with full power applied. It s conceivable that a pilot flying a real slow approach might apply full power to go around and raise the nose too quickly before the airplane has a chance to accelerate to climb airspeed. The pilot notices that the airplane is still descending, even with Thrust Horsepower #3 #2 Maintaining altitude with 2,600 RPM at 53 knots 40 #1 full power. He raised the nose further to arrest the descent and actually increases the descent rate. The pilot s only option in this instance is to lower the nose and allow the airplane to accelerate. Once the airplane is in a region requiring less power to maintain altitude, it can use its excess power to climb. Suppose you re on an approach to landing and your airspeed is deep within the region of reversed command (close to stall speed). In this condition, you must not attempt to stretch a glide by raising the nose without adding power. Doing so will only increase your rate of descent. Many pilots have fallen victim to the region of reversed command. Think of it as licking honey off a thorn you could get stuck if you re not careful. Operating in this region is perfectly safe as long as you re aware of these principles. ut what happens if your engine quits and you have no power? What s the best way to obtain the airplane s maximum glide distance? To find out, we need to look a little closer at the best L/D speed (see Postflight riefing #2-4). THE REGION OF REVERSED COMMND REGION OF REVERSED COMMND (Operating behind the power curve) Descending 500 FPM with 1,900 RPM at 53 knots Fig. 71 Power In the region of reversed command, decreasing airspeed requires an increase in power to maintain altitude or an established descent rate. Maintaining altitude with 1,900 RPM at 74 knots required to maintain altitude Fig. 72 The airplane at position #1 is at the bottom of the power curve and is maintaining its altitude. s it slows down to 53 knots (position #2), it falls into the region of reversed command (below the power curve) and starts to descend. It needs more power to maintain altitude. t position #3, power is added and the airplane can now hold its altitude irspeed in Knots

43 Chapter 2 - erodynamics: The Wing is the Thing 43 Postflight riefing #2-4 Weight, Glide & the Ride The est Glide Speed nd Weight Changes What happens to our glide distance as the weight of the aircraft changes? Strangely enough, a change in weight doesn t affect the glide ratio as long as you make a change in glide speed. To obtain the best L/D of which the airplane is capable, a decrease in weight requires a decrease in airspeed. Let s see why. The best glide speed of a particular Cessna Cardinal 177 is 74 knots at its maximum gross weight of 2,500 pounds. This speed gives the airplane a maximum glide ratio of 10:1. The L/D ratio can also be interpreted to mean that for every 10 pounds of lift developed by the wings, the entire airplane produces one pound of drag. t the airplane s maximum gross weight of 2,500 pounds (thus developing 2,500 pounds of lift), it produces 250 pounds of drag at its best glide speed of 74 knots. ssume the Cardinal weighed 2,500 pounds before you took your flight bag out. Now it weighs 2,000 pounds (that s one heavy flight bag!). Only 2,000 pounds of lift are necessary to equal the airplane s weight in a descent. t 2,000 pounds of lift, an L/D ratio of 10:1 results in the airplane producing only 200 pounds of drag (remember, the L/D ratio is the best that specific airplane is capable of doing). Therefore, if 74 knots of airspeed produces 250 pounds of drag, it stands to reason that a slower airspeed would produce less drag (up to a point, of course). s the airplane s weight decreases, its airspeed must also decrease to maintain the best L/D. Engineers tell us that an airspeed of 66 knots produces 200 pounds of drag in the Cardinal. t this slower speed, our L/D ratio is still 10:1 (2,000/200 = 10/1). t this lower weight, any speed other than 66 knots produces more than 200 pounds of drag which would decrease our 10:1 glide ratio (a shift of the total drag curve resulting from a change in weight). s the airplane s weight decreases, the speed to maintain the best L/D also decreases. s long as the pilot keeps the airplane at the airspeed corresponding to the lowest part of the total drag curve, the L/D ratio remains constant. ut how do you know what the best glide speed is if it varies with weight? Refer to the POH or to the owner s manual. Remember, in the event of a power failure, immediately establish the proper attitude to obtain the best glide speed. Figure 73 provides information from a typical pilot handbook depicting several glide speeds for different weights. Remember, regardless of weight change, if the airplane is flown at the best glide speed for that weight, it will always glide the same maximum distance it s capable of. Keep in mind that most of the glide charts in your POH stipulate conditions like LIFT/DRG SPEEDS & VRILE WEIGHTS Weight irspeed for est L/D lbs. Knots - IS 2, Knots 2, Knots 1, Knots est L/D 68 kts. est L/D 62 kts. est L/D 56 kts. Fig. 73 2,550 lbs lbs. 1,750 lbs. prop windmilling, no wind, flaps up. ny deviation from these conditions changes your glide distance. Most small general aviation airplanes have L/D s somewhere around 10:1. In other words, you can glide approximately 10 times the distance of your height above terrain (assuming flat terrain). This is a typical glide ratio for most smaller airplanes. Gliders, on the other hand, have L/Ds upward of 40:1 or 50:1. (That s why they call them gliders. Some aircraft, like the space shuttle, should be called sinkers for obvious reasons.) Do birds fly? HOW WE USE EXCUSES TO COMPENSTE FOR GLIDE PROLEMS OK ob, can you put her on the numbers? No, I meant the numbers you first flew over! Oh. 30 N2132

44 44 Rod Machado s Private Pilot Handbook Postflight riefing #2-5 Different Look at Maneuvering Speed The first time you encounter turbulence I know what you re going to do. You ll peek out the right then the left window to make sure the wings are OK (as if you wouldn t know). Fair enough. Even though the wings are subject to lots of stress, you needn t worry about them breaking as long as you do one thing. Simply keep the airplane at or below its design maneuvering speed in turbulence. Here s how this works. The design maneuvering speed (Va) is the speed at which the airplane will stall before exceeding its design limit-load factor in turbulent conditions or when the flight controls are suddenly and fully deflected in flight. Under these conditions the airframe experiences an increase in G-force or load factor (we ll use these terms synonymously). The limit-load factor of U.S. certificated airplanes is based on the maximum amount of G-force the airframe can withstand before becoming damaged. irplanes stressed up to but not beyond their limit-load factor should experience no structural damage. (This assumes the airplane is like new and not previously overstressed.) For the purposes of airplane certification, airplanes are certified in one of three categories: normal, utility, aerobatic. The stress limits for these three categories are: +3.8Gs and -1.52Gs for normal category airplanes; +4.4Gs and -1.76Gs for the utility category airplane; +6Gs and -3Gs for the aerobatic category airplane. Let s examine how maneuvering speed prevents us from exceeding these limits. Standing anywhere on planet Earth, you ll experience one time the force of gravity or 1G. It s gravity that pulls you toward Earth s center, providing the feeling of weight. On the surface, gravity exerts a constant 1G pull. In an airplane, however, you and the airplane can feel like you weigh more than your actual weight. This occurs when the airplane turns or the angle of attack suddenly increases (as it does in turbulence). This increase in apparent weight is called an increase in G-force or load factor. Let s suppose our airplane is cruising in straight and level flight at a constant airspeed. In this condition, lift is equal to weight and we experience a G-force of 1. We can represent G-force by the formula Lift/Weight = G-force. In straight and level unaccelerated flight the lift developed by the wings is equal to the airplane s weight. Thus, the G-force is 1G. If the angle of attack suddenly increased (by pulling back on the elevator or encountering a vertical gust of wind for example), the wings would produce an instantaneous increase in lift. The airplane accelerates upward and you re forced downward in your seat. In other words, there is more upward pull by lift compared to the downward pull by weight. The G-force increases proportionally to the sudden increase in lift. Instantaneously doubling, tripling or quadrupling the lift doubles, triples or quadruples the G-force. direct (or nearly so), one-to-one relationship exists between lift and angle of attack. For instance, at a constant airspeed, in a 1G condition, a sudden doubling of the angle of attack doubles the wing s lift and doubles the G-force. Tripling or quadrupling the angle of attack triples or quadruples the G-force. For instance, assume the airplane is flying at a fast cruise speed of 140 knots as shown in Figure 74. The airplane and its contents experience 1G at an angle of attack of 3 degrees for level flight. Suppose a sudden gust increases the angle of attack by an additional 3 degrees. The wing s original angle of attack has now doubled to 6 degrees (3+3=6). Consequently, lift suddenly doubles, producing 2Gs. sudden increase in angle of attack to 9 degrees triples the lift and the G-force. n increase to 12, 15 and 18 degrees increases the lift and G-force to four, five and six times its original value. Fig ngle of ttack t 140 knots indicated airspeed with an abrupt pull-back on the elevator or an encounter with a strong vertical wind gust. 1G 2Gs 3Gs 4Gs 5Gs ngle of ttack 9.0 ngle of ttack ngle of ttack ngle of ttack 6Gs ngle of ttack Original angle of attack in cruise flight at 140 knots Sudden increase to twice the original angle of attack Sudden increase to three times the original angle of attack Sudden increase to four times the original angle of attack Sudden increase to five times the original angle of attack STLL Sudden increase to six times the original angle of attack.

45 Chapter 2 - erodynamics: The Wing is the Thing 45 Since the wing stalls (and lift production decreases) at an angle of approximately 18 degrees, any further increase in angle of attack beyond six times the original value of 3 degrees won t increase the G- force. There-fore, at 140 knots this airplane is capable of experiencing 6Gs before the wings stall. If the airplane in this example had a limit load factor of 4Gs, the structure might experience some damage at this speed in strong turbulence. (I chose ngle of ttack Original angle of attack in cruise flight at 110 knots 4Gs instead of the actual limit loads of +3.8, +4.4 or +6Gs for this example to simplify the math.) If we slow our airplane down to 110 knots (Figure 75), then let s assume a 4.5 degree angle of attack is necessary for a 1G, level-flight condition. If, at 110 knots, we suddenly double the original angle of attack to 9 degrees, the lift doubles. We now feel 2Gs. sudden tripling of the original angle of attack to 13.5 degrees, triples the lift and we experience 3Gs. nd finally, quadrupling the original angle of attack to 18 degrees produces four times as much lift. Therefore, we experience 4Gs. It s not possible to pull more than 4Gs in this example since the airplane will stall at 18 degrees. Consequently, 110 knots is the maneuvering speed for our airplane having a limit-load factor of 4Gs. t 110 knots, the airplane will stall before it exceeds this limit load factor in turbulent air or with full deflection of the flight controls. (Personally, in turbulence, I prefer to fly 10 to 15 knots below Va to prevent a gust from temporarily raising my indicated airspeed above Va.) Whether a specific gust doubles, triples or quadruples the angle of attack, depends on the angle of attack of the airplane in its 1G condition. s is clearly evident from these examples, it s easier for a gust to double, triple or quadruple the angle of attack over its starting value when the airplane is flying faster (because it s at a lower angle of attack to begin with). Consequently, it s easier to experience more Gs for a given amount of turbulence at higher airspeeds. Weight Change and Va The airplane s posted maneuvering speed (Va) is based on the airplane being at gross weight. What happens when the airplane s weight decreases? The answer is, the maneuvering speed decreases. Let me explain. irplanes flown at weights below their gross weight require less lift for straight and level flight. Less lift means the airplane can be flown at a smaller angle of attack. In other words, an airplane at 2,500 pounds may require a 4.5 degree angle of attack at 110 knots to remain in level flight. Decreasing the weight to 1,800 pounds may require only a 3 degree angle of attack to t 110 knots indicated airspeed with an abrupt pull-back on the elevator or an encounter with a strong vertical wind gust. Fig. 75 1G 2Gs 3Gs 4Gs ngle of ttack Sudden increase to twice the original angle of attack ngle of ttack Sudden increase to three times the original angle of attack ngle of ttack STLL Sudden increase to four times the original angle of attack remain in level flight at this speed. Does this angle of attack sound familiar to you? With a speed of 110 knots, at this lower weight, a sudden and very strong gust could increase the angle of attack from 3 to 18 degrees. From our previous example, this produces six times the original lift for a force of 6Gs. This is way beyond the limit of a normal category airplane. t lighter weights, what can we do to keep from exceeding our example limit of 4Gs when in turbulence? The answer is to slow the airplane down. t a slower speed (95 knots for example) a larger angle of attack (let s say 4.5 degrees) is necessary for level cruise flight at this lower weight. t this speed, we can increase the angle of attack four times before the airplane stalls. Ninety-five knots becomes our new maneuvering speed if we want to limit ourselves to 4Gs. Thus, decreasing weight requires a decrease in the airplane s maneuvering speed. Most of the newer pilot s operating handbooks publish two or three different maneuvering speeds for variable weight conditions (these are now called Vo s or operating maneuvering speeds, while the term Va refers to the maneuvering speed for max weight). If yours doesn t, try doing the following to compute a new one. For every 2% reduction in weight, reduce the max-weight maneuvering speed by 1%. In other words, if the gross weight decreases by 20%, reduce the max-gross weight maneuvering speed by 10%. This is simple math. (esides, don t feel too bad if you are confused about math. For many years, I thought the logarithms was a singing group at MIT.) few final words. If you encounter turbulence in flight, fly a level flight attitude. ttempting to hold altitude at the expense of maintaining the proper airspeed might overstress the airplane. For those engineers reading this, the airspeed-angle of attack values are approximations only. I also assumed that the zero-lift angle of attack is zero degrees. linearlift curve is also assumed with a lift coefficient that increases 0.1 for each degree angle of attack increase (this is close enough to the real world for our purposes).

46 46 Rod Machado s Private Pilot Handbook Postflight riefing #2-6 erodynamic Ideas for Your Consumption Rectangular Moderate Taper Elliptical Pointed Tip Swept STLL PTTERN PROGRESSION Wing design has an effect on how the airflow separates from the wing during a stall. The rectangular wing has the advantage of stalling at the root first. This keeps the ailerons effective at high angles of attack and provides you with a good stall warning buffet. moderate taper on the wing allows more of the wing to reach a stall at the same time. This tends to reduce aileron effectiveness during a stall. ll sections of the elliptical wing reach the stall at the same time. ilerons may lack effectiveness during the stall on this wing. Pointed and swept wings stall at the tips first, then progress inward, rendering the ailerons ineffective during the stall. Some airplane manufacturers place "stall strips" on the wing's leading edge to ensure a specific part of the wing stalls first. Usually this is the inboard section of the wing. This keeps the ailerons effective during the stall. This means that the pilot still has roll control when experiencing the prestall (buffet) warning signs. STLL STRIPS Stall Strip FROST, ICE OR SNOW THEY MUST GO Courtesy NS Don t Even Think bout It! Frost on the airfoil disrupts the smooth flow of air over the wing. This causes earlier airflow separation resulting in decreased lift and increased drag. While the airplane may accelerate with frost on the wings, it may be impossible to achieve enough lift to become airborne. Frost also increases the stalling speed of the airplane. If a pilot does manage to become airborne, the airplane is now more likely to stall with any increase in load factor or decrease in airspeed. lways remove all frost and ice from the airplane before takeoff. Usually it must be brushed off. Squirting down the airplane with a hose isn t a good idea either! Remember, frost forms when the temperature of the collecting surface is at or below the dewpoint of the adjacent air and the dewpoint is below freezing. Shooting water on the wings may simply increase the amount of frost and ice already present. C Photos courtesy of Captain arry Schiff Photos courtesy of Captain arry Schiff Photos courtesy of Captain arry Schiff Seeing the Stall Stall progression is clearly seen with the placement of small strands of yarn taped along the wing. In picture, air flows smoothly over the top of an unstalled wing. Picture shows the progression of a stall from the wing root outward as the wing approaches its critical angle of attack. This is usually felt as a slight buffet which pilots know to be one of the early warning signs of the beginning of a stall. Picture C shows the wing in a complete stall. Notice how the yarn on the inboard section of the wing has actually reversed direction. This shows how the airflow over the wing becomes turbulent, no longer producing lift efficiently. Most general aviation airplanes have some variant of a rectangular-tapered wing. This allows the stall to progress from the wing root outward, allowing the ailerons to have some effectiveness for roll control as the stall is approached. This can be seen in C, above.